I completely agree that LoL doesn't need more maps. I don't quite have time to explain my whole thought process behind this, but here are a few points real quick.
First of all, the concept of maps getting boring. When you include multiple maps into a game, favorites get chosen quickly. With how much maps effect this game, and how hard it would be to balance them, its unlikely we'll ever have more than say 5 real maps. With such a small map pool, the favorite will limit the game back to one map, and all the work on other maps has been wasted.
More maps would also be a balancing nightmare, if Riot continues to balance how they do now- b fixing everything as it comes up and making sure nothing is "FotM" for too long. Balancing like this on multiple maps would be almost impossible.