That is a good point, it does hurt creativity in the meta-game. Though it also prevents possible troll-type situations (all Engineers, all Spy's in TF2? A server I was on put a class limit for this various reason)
Btw, on this subject, LoL just announced they are adding in support for strict team comp (I think as an option, not a requirement):
Lyte here, and in this post I’m excited to talk with you about Team Builder, a new feature coming to PBE in the near future where you can queue up for a specific role, champion and position and get connected with four teammates that also preselected their preferred role. The goal is to build a balanced team where everyone’s in agreement on where they’re going and what they’re playing, and in doing so help lay the foundation for good teamwork.
edit: Looks like this is more flexible than I was thinking, they will even support things like double jungle etc. So it is more along the lines of something to make Matchmaking easier (eg, make it a better user experience for you to get the roll you want, without really limiting the types of meta too much).