i'll have the time this weekend to finally begin reading through threads but i wanted to just pose questions to the OP, sorry in advance if this discussion took a turn already toward these questions (i see people complaining about a smogon/gamefreak debate though, which i'll clean up when i get home tonight)
Why? What is inherently more skillful about knowing you don't have to worry about one more threat after it's been revealed than having to judge the likelihood a threat is carrying a specific item, regardless of what you've seen so far. I could easily argue there is just as much skill in making educated guesses of the opponent's team based off of information you already know ahead of time or thinking about what they could potentially have and how to play around it (I've seen two scarfers already, so I can try to predict this mon isn't scarfed; I've seen Choice Band on a mon I didn't expect already, how can I play around multiple Choice Band Pokemon; etc) instead of having that information just handed out to me once I see a Pokemon. If the argument is "maximizing the potential of your items", why doesn't this apply currently to Pokemon? We still have to decide between which item maximizes the effectiveness of our mon, but we're making the decision between more useful items such as Leftovers, Life Orb, Assault Vest, etc for every Pokemon.
The same reason that team preview provides better play. It means that the player can work to not only take out key Pokemon threats, but also key item threats.
How? Even though you're limiting Leftovers and other defensive items, you're limiting Life Orb as well, one of the key items to a "faster" paced style, as well as Choice Scarf. Why is faster paced better in the first place? If skill is determined by the strength of your play, and the lack of your misplays, then wouldn't logic dictate that the longer a game goes on, the better chance skill plays a larger role by increasing the number of turns one person has to misplay?
My own personal opinion, I enjoy faster paced games. The whole, "Item Clause = more skillful play" refers to working out what items your opponent's team has and being able to counter them. Ever played Cluedo? I expect working out items would be something like that.
Why? We talk about how diversity/variation is good, but what exactly makes them a true showing of competitiveness. In Yu-Gi-Oh!, the one-deck formats have proven time and time again to normally be the most skillful formats. So why in Pokemon do we believe having more options available to the player makes the game more competitive? This is a question that needs to actually have an answer, because otherwise the suggestion is limiting item use for no reason whatsoever.
My own personal opinion. I find the over centralised nature of Yu-Gi-Oh boring, there's nothing better than seeing an original deck doing so well against what has been tried and tested as being "the best." For a Pokemon example, see Chainchomp.