The main reason to have more levels in the pool is the fact that the new generation of players should see what awesome levels are out there - even if some games get a bit unbalanced.
Cuz ATM ain't no one out there that will play a non pool game with you. In general of course.

As for 3v3' levels being too big and too slow, I find you contradict yourself on that point gry. Sbv2, marsh and zaitox and some other legendary levels that aren't competitively playable in 2v2 games could be added for clanwars or 3v3/4v4 games.

New levels are always appreciated as well. Diversity might bring about some problems, but that doesn't mean we shouldn't try, and maybe push a level or two in there even if teh balance isn't completely right.
Hell our last DW game, we got 2 rushes each worth 2 points and it's still in the pool.