Well Eve at most as about 35,000 people online at once. I think the record was 38k last year for them. And Eve has about 7,000 different regions of space for those players to spread out to.
Swtor has almost 2 million players so far and far less zones than Eve. Now some games like Star Trek Online did the single server thing but made each zone only hold 50 players per shard. That was awful because you were always stuck away from group mates and friends.
Different servers allow some different play styles also. Like PVP, RP, PVE. and can have language specific servers also.