I find it annoying and pointless. If people are still running 6 Lefties, they're really just handicapping themselves. Life Orb, the three Choice Items, all the berries, Focus Sash, etc; all lead to some pretty nice strategies that can't be run otherwise.
But you do end up in some area where you'll want two or three pokes to pack Lefties. Generally people pack atleast two tanks, a special and physical one. It makes sense here to put Lefties on them since its a great job enhancer. Using Item Clause just forces people to have stupid things like Shell Bell, Lax Incense, and stuff that generally aren't as good.
As stated earlier, if you want to maximize your teams potential under Item Clause, you'll fall into a basic formula of making sure you have a Berry user, a Lefties Tank, a Life Orber, Multiple Choice users. This is really a needless clause that if anything forces people to use even more "standard" teams.
The game is much more diverse if people are allowed to pick their own items, even if they run with duplicates.