Thank you all for replies!For now uniqueness is not only reason for isometric view.The game client is planned to be in browser (webgl-based), and it's my first webgl game, so I doubt that we will be able to make good FPS graphics in webgl. I hope that I am wrong.
The third reason is that I think game will look more epic (especially on full server) in isometric perspective.
Oh, then seems we are on similar boats (browser based as well , though not webGL but Canvas) , wish graphics were the sole or biggest problem in a fast paced browser game.