My standpoint: Against. And here's my 2-page (I think) essay as to why:
Firstly, everyone who's pro-clause is saying: "OH IT WILL ENCOURAGE MORE DIVERSITY IT MUST BE GOOD :DDDDDD" Nay-nay. Not all diversity is good. Let's take some examples about how "good" diversity is from real life, and Pokemon as well. There are many illicit (illegal) drugs. By your logic, you're saying that the diversity of these drugs and how they are abused is good. Pretty strong logic there. Now let's apply this diversity argument to Pokemon, and see why you're wrong. Let's say you use NU pokes in the new OU, such as Pikachu. By your logic, it's the only one to hold Light Ball, therefore you have other slots free for more items on other Pokemon, making the Item Clause viable, allowing you to put a Choice Scarf revenge killer.
WRONG. That Pikachu dies to almost every hit in the game, meaning you just wasted a team slot that could've had another Choice Scarf'd revenge killer on it, for instance. How about Hippopotas with Eviolite? Leftovers Hippowdon outclasses in every way, shape, and form, besides freeing up Leftovers. These Pokemon which you're trying to make usable through item clause are not used as often for a reason! Besides, this diversity will eventually fade away as threats are named, just like it does in OU today.
Counter-Argument: But it will encourage more people to be creative!
SO WHAT? And just like that, we're back to the diversity point, not all creativity is good. See the Silly things you've seen on the X Ladder threads to show just how "good" creativity is. Besides, it's not exactly like people still won't use Smogon sets, adjusting for Item Clause :\
Counter-Argument 2: But it will encourage more people to be strategic! Really? You call slapping a different item on an already existing set strategic? How about just making a random set that wouldn't be viable at all just to use the item you are putting on the set now? Yeah, that's definitely strategic :\
Do not make the wrong assumption that introducing item clause decentralises the meta-game. The focus merely shift away to another item meta-game. I have yet to see anyone make good arguments about why item clause causes decentralisation.
Leading off into my second point with this quote right here. Variety ≠ Decentralization. You will still have Banded Scizor, Scarf Keldeo, etc. being used, but everyone else will have a different item. If anything, because of the unviability of some of the sets with those different items, the metagame will be
even more centralized. So why? You need to figure out which Pokemon can use the item most effectively. Choice Band is best used on Scizor, an already used-a-heck-of-a-lot Poke. MOAR CENTRALIZATION! Choice Scarf? Latios is pretty good with one, also being able to Trick it, so it's one of your best choices. MOAR CENTRALIZATION! Rocky Helmet, so you can free up lefties? Your Ferrothorn could sure use more residual damage on contact moves. MOAR CENTRALIZATION! These are just a few examples, but there are many more that shows that it actually centralizes the meta even more. Besides, even if it somehow does decentralize the meta, it will return to equilibrium at some point where some Pokemon are used more than others, which is, as you probably already guessed,
CENTRALIZATION.
So here's my 2-paragraph essay on how item clause should not be implemented. I probably missed a lot of points, because I don't want to absolutely fill up the screen or anything (probably already did :\).