Setting caps that server owners can't change is a bad idea, not every server will always be for 64 or more players only, and I'm sure those extra slots will come in really handy for modifications. I can see the extra slots, and even go above 100+ for bots and specific game modes, etc.
Ultimately its up the the players anyways, if there is a server with (lets pick a insanely high number - 200+) people in it, other players always have the choice not to join, yet still allowing for those who might want to try this extremely odd game.
I don't see why we would implement a mandatory cap of anything.