If most exchanges keep coins in a separate address for each user, then they will control the vast majority of clams. Most users probably keep all their coins in one address if they run a wallet on their computer.

Good practice says you should use a different address for each incoming payment -- even from the same person. A lot of people (including myself) don't follow through with that 100% of the time. But those that do will benefit the most from the initial distribution. I think that's an awesome thing, since it helps promote security.