It's extremely rare that I see a thread like this where I can't help but disagree with the side I'm on. I don't believe there should be an item clause but I say that out of personal preference. I don't believe it's either a superior choice, nor the most simplistic. In my eyes, the current way Smogon plays is nowhere near the simplest, nor the "best". It's very apparent that this way of playing is not what most people will play, nor what TPCi wish to be most commonly played. For example:
Smogon plays at level 100 or less. Wifi enforces level 50.
Smogon plays with a ban list (or many, depending on tiers) designed around usage. Wifi enforces only non-specials.
Smogon allows multiple items. Wifi enforces item clause.
These are just the major differences. There's no use saying "it will kill x playstyle" or "it will make the game more varied" because frankly, there's no way to know. The game played here is vastly different to what the majority of people will play in game. This is akin to trying to tier pokémon before anyone has used them in ten dozen battles; it's mindless speculation. Many of the arguments being put forth show little to no understanding of why things are done in the way they are, with many clearly just not wanting to change. You cannot say the item clause doesn't stifle a small amount of creativity, because it does - many stalling concepts rely on passive recovery from items. Likewise, it's foolish to say the lack of item clause allows more pokémon to be used - that's highly unlikely and there's no evidence to back it up; if VGC has taught me anything it's that people will adapt to clauses and if I'm honest, VGC is incredibly diverse.
Ultimately, the game is like this because this is the way we choose to play. That's it. There's nothing else to it. We cannot say a player at the top of the Smogon ladder is better than one topping wifi tournaments in singles - the differences make it almost totally incomparable. It's not extra diversity, it's not more simplistic (in fact, it's probably far easier to just adhere strictly to wifi rules as then everyone is playing the same thing) and it is absolutely not for the sake of trying to create the "best" metagame possible - to think that is to be exceedingly naïve. Instead, the game is like this because the majority of people here have chosen to play in a slightly more restricted way, how Smogon plays (and yes, I would definitely say the restrictions on OHKO moves, evasion, the pokémon clause, ability bans, tiering, etc is more restrictive than wifi has ever been), not out of some great scheme or attempt to create the greatest variety, but merely to have fun and try to inject some semblance of balance into a game. That's it really - every single thing which has been banned has been for the sake of balancing a game. To do so, the restrictions are added onto a list. I won't even begin to get into the level debate, but honestly it's almost irrelevant in terms of balance and changing now would only serve to invalidate many previous calculations. Let's not pretend otherwise - Dialga isn't banned because someone found it ugly, Giratina wasn't banned for the sake of making people change teams for the sake of it - it's in the name of balance. Now, we can argue to dawn whether this has been successful, but it begs the following.
All bans have so far been made either due to balance or lack of attainability. Why then would we want to enact something which will have inconsequential effects on balance when that's never been done before?
So that's my view. Were Smogon to change its purpose and instead try to emulate TPCi's battling form, then yes, an item clause should be considered. However, it hasn't so far changed due to wifi changes, so why begin now? It seems pointless. I'd like to see a vote on whether people would play in a separate singles tier with an item clause, but to enact one on all battles just for the sake of it seems somewhat redundant.
That said, whilst I'm here I'd like to address qsumbreon's points on VGC, because they bug me. In no way should item clause use be decided due to any VGC evidence anyway as it's doubles not singles, but this needs to be addressed as it's almost entirely wrong.
1) It depends on what you mean by less switching if I'm honest. In many games, especially higher level play, switching is utterly crucial in VGC and it's not unusual to see more switching per turn than singles. Entry hazards are poor as there's no way to set them up in a way that inflicts a lot of damage unlike singles, and uses up a valuable slot in a game where you only get 4 to begin with. Now, it's never going to be the case that it'll rival a stall team in singles, but the general idea that VGC has little switching is at best incorrect and at worst harmful to new players in the tier. Realistically, the only time there's not much switching is when one person is playing a hyper offensive team.
2) Never have I read a point that misjudged VGC so badly. VGC if anything was far more varied than OU at the end of gen V and cannot possibly be reasonably described as stale. This is the same meta that had the UK champion almost win UK nationals with a Gary themed team; had hail, trick room and tailwind as a viable team basing and where EV spreads were incredibly varied (224 HP / 72 Def / 136 SAtk / 76 SDef / 4 Spe Cress, 40 HP / 48 Def / 128 SAtk / 104 SDef / 188 Spd Thundurus, 252 HP / 116 Atk / 4 Def / 136 SpDef Metagross to name a few). You would never see those things in top end singles teams unless they were commanded by a very stubborn, high level player - they just don't function very well in singles. Likewise, spreads which help bulk are, on the whole, worthless in singles where most battles depend on X entry hazards doing flat damage and hitting first can neuter all opposition, which isn't the case in doubles where the partnered pokémon can strike and multi-targeting moves exist. I can only assume you didn't play it much honestly, or assumed that variety only comes from the pokémon used (though I'm fairly sure there's also more different pokémon used as well) and not also sets, spreads and team types.
3) VGC doesn't have item clause to inhibit stall, it has it because every single format TPCi has ever run has had item clause. Indeed, stall is alive and well in VGCs, with one of the top players going so far as to RNG shinies only so that the extra frames of animation counted down the clock a little bit to enable a stalled win on time. Whenever a system has timed battles, you will see stall - VGC is no exception, especially with no evasion clause and protect being on many things. Just because the end game for stall isn't killing everything doesn't mean it is absent. Again, I can only assume you didn't play much as if you had, you'd both know how long VGC events go on for and that the timer is in all battles to prevent overrunning massively. This has nothing to do with the item clause choice.