I really don't think there's a necessity for the implementation of Item Clause. Honestly, it seems like a lot of people just like suggesting/demanding bans and limitations for the sake of them. The past few weeks especially have had a bunch of people ask for clauses and bans without sufficient reasoning for them; here's a few links to make that point clear.
http://www.smogon.com/forums/threads/item-clause.3490570/
http://www.smogon.com/forums/threads/evasion.3490307/
Now, the OP asks two distinct questions. The first of those is "would an Item Clause make GSC more popular?" This it totally separate from the question of whether it make the GSC metagame better.
Would it make GSC more popular? Probably not. If anything, I see it making the game less popular. It would probably drive away a bunch of people who prefer it played without such a clause in effect. In terms of new players, who is it going to draw in? I don't think there's a real demographic of people who are thinking "gee, GSC sure looks cool, but I just can't get over the fact that multiple Pokemon can hold the same item like in basically every other metagame." GSC has other reasons for not drawing people in.
On a related note, why does GSC need to be more popular? I'm sure a lot of people who play GSC are fine with not having as many players as, say, Gen VI OU has right now. Plus, popularity doesn't equal quality. Gen VI has lots of players, but you still have to fight mono-Fairy teams and other non-competitive stuff from time to time. I don't have any real experience in GSC, but I'm sure you don't encounter such things by virtue of not drawing in that crowd.
The other question the OP brings up is "would the GSC metagame be better with an Item Clause?" To argue my points I'm going to use the Characteristics of a Desirable Metagame thread that was brought up earlier. While I know it's not the end-all-be-all to this kind of debate, I think it's more than sufficient to help argue against implementing an Item Clause.
So, the metagame should, ideally, be competitive, varied, balanced, stable, skill-based and efficient while adhering to the rules and spirit of the original games and allowing some luck. I don't think an Item Clause merits discussion in the realms of competition, stability or efficiency, so we'll ignore those. At the risk of making this post even wordier than it already is, I'm gonna break down the remaining characteristics a bit more.
Variety: Variation in items does not equal variation in playstyles and Pokemon used. In later metagames you can see that not limiting items arguably leads to more variety with the rise of things like teams that run 6 Choice items in Gen V (whether or not that's good for the metagame is another discussion entirely.) While some Pokemon are massively affected by their item choices (meaning Marowak,) most of them don't become entirely different beasts with new items. Let's pretend we had a bunch of different Snorlax with the items King's Rock, Scope Lens and Quick Claw. While you have three item choices here, they don't make a meaningful difference in how these Snorlax are played. They all attack as usual and hope the item's effects come into play.
Just because this is where I think it'd fit best, here are a few damage calculations that help show how little the type-enhancing items change things. I'm using Honkalculator, but set the stats and such to GSC levels.
Balance: The ubiquity of leftovers is definitely not the primary reason GSC is so stally. Maxed stats and much better recovery options for basically every Pokemon are why the game is so stall-centric. Notice that leftovers work exactly the same in the next 4 generations, but none of them are nearly as stall-based. Nearly any Pokemon can run a RestTalk set in GSC or even just Rest without Sleep Talk.
For that matter, GSC doesn't need to shift away from stall. The game is still (arguably, at least,) varied and balanced while being stall-centric. Can you go hyper-offensive and break the opponent down in seconds? No. That doesn't mean that highly offensive Pokemon like Scizor aren't viable though. They just work differently. GSC gives you options (albeit there are certainly problems here and there, like in any metagame,) and you aren't really making more options/balancing preexisting options with an Item Clause.
Skill/Luck: I'm gonna discuss these together. The basic argument here is that limiting item choices isn't going to make the game more skill-based, and luck will probably rear its ugly head more than it should. I'm of the opinion that Skill and Luck are mutually exclusive, and while I accept luck as part of the game, I definitely wouldn't play Pokemon competitively if every team ran Brightpowder and Quick Claw.
Adherance: The OP argues that Item Clause should be implemented to be similar to the few official tournaments for GSC. The problem with this argument is that we aren't aiming to emulate official tournaments; we're just emulating standard cartridge battles. When the rules for VGC '11 were announced, do you think anyone was saying "hold the phone, why are simulators allowing non Gen V Pokemon on teams, that's not allowed in the tournaments!"? VGC '11 was treated as a separate metagame, while the "main," metagame simply allowed what's allowed in-game. If GSC ever gets popular enough to warrant a separate metagame for old tournament rules, that's great. At the moment it does not though, and those rules shouldn't be emulated in the main GSC metagame.
So, to the OP, I don't see how implementing an Item Clause is a "brainless decision," in it's favor. If anything, all signs point the other way around. Don't get me wrong, I'm not trying to sound condescending or anything here. I love a good debate. I just don't see a reasonable argument in favor of Item Clause.
http://www.smogon.com/forums/threads/item-clause.3490570/
http://www.smogon.com/forums/threads/evasion.3490307/
Now, the OP asks two distinct questions. The first of those is "would an Item Clause make GSC more popular?" This it totally separate from the question of whether it make the GSC metagame better.
Would it make GSC more popular? Probably not. If anything, I see it making the game less popular. It would probably drive away a bunch of people who prefer it played without such a clause in effect. In terms of new players, who is it going to draw in? I don't think there's a real demographic of people who are thinking "gee, GSC sure looks cool, but I just can't get over the fact that multiple Pokemon can hold the same item like in basically every other metagame." GSC has other reasons for not drawing people in.
On a related note, why does GSC need to be more popular? I'm sure a lot of people who play GSC are fine with not having as many players as, say, Gen VI OU has right now. Plus, popularity doesn't equal quality. Gen VI has lots of players, but you still have to fight mono-Fairy teams and other non-competitive stuff from time to time. I don't have any real experience in GSC, but I'm sure you don't encounter such things by virtue of not drawing in that crowd.
The other question the OP brings up is "would the GSC metagame be better with an Item Clause?" To argue my points I'm going to use the Characteristics of a Desirable Metagame thread that was brought up earlier. While I know it's not the end-all-be-all to this kind of debate, I think it's more than sufficient to help argue against implementing an Item Clause.
So, the metagame should, ideally, be competitive, varied, balanced, stable, skill-based and efficient while adhering to the rules and spirit of the original games and allowing some luck. I don't think an Item Clause merits discussion in the realms of competition, stability or efficiency, so we'll ignore those. At the risk of making this post even wordier than it already is, I'm gonna break down the remaining characteristics a bit more.
Variety: Variation in items does not equal variation in playstyles and Pokemon used. In later metagames you can see that not limiting items arguably leads to more variety with the rise of things like teams that run 6 Choice items in Gen V (whether or not that's good for the metagame is another discussion entirely.) While some Pokemon are massively affected by their item choices (meaning Marowak,) most of them don't become entirely different beasts with new items. Let's pretend we had a bunch of different Snorlax with the items King's Rock, Scope Lens and Quick Claw. While you have three item choices here, they don't make a meaningful difference in how these Snorlax are played. They all attack as usual and hope the item's effects come into play.
Just because this is where I think it'd fit best, here are a few damage calculations that help show how little the type-enhancing items change things. I'm using Honkalculator, but set the stats and such to GSC levels.
That's only a couple of examples, but that's not changing 3HKO's into 2HKO's, and those are some really powerful attacks (in the vs. Raikou examples he's Ground-weak and it barely matters.)
Balance: The ubiquity of leftovers is definitely not the primary reason GSC is so stally. Maxed stats and much better recovery options for basically every Pokemon are why the game is so stall-centric. Notice that leftovers work exactly the same in the next 4 generations, but none of them are nearly as stall-based. Nearly any Pokemon can run a RestTalk set in GSC or even just Rest without Sleep Talk.
For that matter, GSC doesn't need to shift away from stall. The game is still (arguably, at least,) varied and balanced while being stall-centric. Can you go hyper-offensive and break the opponent down in seconds? No. That doesn't mean that highly offensive Pokemon like Scizor aren't viable though. They just work differently. GSC gives you options (albeit there are certainly problems here and there, like in any metagame,) and you aren't really making more options/balancing preexisting options with an Item Clause.
Skill/Luck: I'm gonna discuss these together. The basic argument here is that limiting item choices isn't going to make the game more skill-based, and luck will probably rear its ugly head more than it should. I'm of the opinion that Skill and Luck are mutually exclusive, and while I accept luck as part of the game, I definitely wouldn't play Pokemon competitively if every team ran Brightpowder and Quick Claw.
Adherance: The OP argues that Item Clause should be implemented to be similar to the few official tournaments for GSC. The problem with this argument is that we aren't aiming to emulate official tournaments; we're just emulating standard cartridge battles. When the rules for VGC '11 were announced, do you think anyone was saying "hold the phone, why are simulators allowing non Gen V Pokemon on teams, that's not allowed in the tournaments!"? VGC '11 was treated as a separate metagame, while the "main," metagame simply allowed what's allowed in-game. If GSC ever gets popular enough to warrant a separate metagame for old tournament rules, that's great. At the moment it does not though, and those rules shouldn't be emulated in the main GSC metagame.
So, to the OP, I don't see how implementing an Item Clause is a "brainless decision," in it's favor. If anything, all signs point the other way around. Don't get me wrong, I'm not trying to sound condescending or anything here. I love a good debate. I just don't see a reasonable argument in favor of Item Clause.