Gimmick sets, perhaps, but there's no guarantee of that. You can't assume that people will assume standard sets any more this way than when they see the Pokemon individually, and certainly not if you're facing an actually good opponent. Furthermore, if the gimmick set involves synergy of any sort, it could be hinted at by the other Pokemon on your team, and thus become even less useful.This would drastically increase the power of gimmick sets/teams. When someone sees your team and prepares for the standard sets and you start breaking out crazy stuff and netting surprise kill after surprise kill, it has to be satisfying.
Gimmick teams, however, are useless under this rule. Absolutely useless. If the Pokemon itself is the gimmick, it'll be horribly obvious what sets they're running and how to counter it.
Really, there's nothing gained in turns of team building options from this. Strategies in battle, perhaps, but that's debatable, and even then, the disadvantages are clear.