How does it stop defensive play, many pokemon have their own move to recover HP, and then there are items like Rocky helmet, Red card, some berries, etc that can help defensive play. Just because you just want leftovers on everything and think that it is the ONLY way to play stall does not make it true.
Yes, you
could use these items on a stall team, but nine times out of ten they would be inferior to leftovers. My SDef Heatran could use a passho berry rather than leftovers, but it would be significantly less effective if it did. These items are niche for a reason and preventing their use is not going to make people use SDef Heatran (for example, don't get wound up over which particular mon I mention) without leftovers, it is going to make people stop using SDef Heatran altogether. There is no concievable way in which this promotes diversity, it just encourages the use of already-popular Pokemon more if they are able to use nonstandard items.
I also find it laughable that the pro-item-clause side wants to bash Stealth Rock while simultaneously trying to resitrict access to what is undoubtedly the single best way of mitigating entry hazard damage. A lot of defensive flying types are really kind of reliant on their leftovers to reduce the impact of consantly switching into Stealth Rock - by forcing them to give leftovers up, you are making the entry hazards you so hate much more powerful.
There just aren't really any valid arguments in favour of this. It doesn't promote diversity, not really, and it doesn't make it easy for Wi-Fi players to adapt to Smogon since we still have an entirely different banlist, event Pokemon and moves and a tiering system they will have to get used to. I mean we get people moaning that legendaries are OP because they got swept by their friends Entei but that doesn't mean we ban legendary Pokemon to make it easier for them to 'adapt' to our metagame.
This is a total tangent, but in response to the idea that tiers limit diversity I have two points. Firstly, no good player discounts lower-tiered Pokemon because they are lower-tiered. They might discount them because they suck, but not because of the tier they are in. Secondly, even even we were to do something retarded like abolishing tiers, things would go from "I'm not using Sharpedo because it's UU so it must be bad" to "I'm not using Sharpedo because it's only used on 1% of teams so it must be bad". You don't change anything.
EDIT:
I might be a fence sitting, but it is this lazy argument which actually draws me more into item clause. "I can't build a team unless every pokemon has leftovers". I mean come-on! Laziness. You just have to create a viable stall that does not rely on that. There are still methods.
This is a really presumtuous argument that completely misses the point of the post it is responding to in a way that seems to me very deliberate. GatoDelFuego is not saying he could not make a viable stall team without leftovers, he is arguing that nobody can. Since the pro-item-clause side is arguing that stall is still viable with item clause active, he is asking them to prove it. And none of them will, because denying stall access to the items it wishes to use in a metagame where defensive strategies are already in quite a precarious position is going to make stall flat-out inviable in high-level play, and that sucks. This isn't a case of "I can't play this strategy without using leftovers as a crutch", it's a case of "this strategy is not realistically viable without access to leftovers for passive recovery and to mitigate hazard damage".