Things don't even out so much as become less noticeable. Group compositions tend to matter a lot less, and it's easier to find the black sheep amongst 1000 players rather than 200. There is no reason you can't have every class have it's own situational strengths and weaknesses, and still be fairly balanced. The thing is you have to design the classes from scratch with that notion in order for it to work. On the other hand, look at blizzard. They created their game without even including any form of instanced PvP. It is clear PvP was not at the forefront of their design philosophy at the time.