I did like the idea of allowing players to take over a cohort.  I think a cohort played by the player of the PC or by the DM is inherently less interesting than one played by a separate player.  If there are players who want to play a cohort, I think we should let them, even if this would give them a 4th character.
A cohort is fundamentally different than a PC, and should be played as such.  A cohort can be full of personality, but obviously shouldn't be a leader.  He/she should be a follower of one PC.  In a way this makes him/her less fun to play, but it is different, and can open the door to some interesting personalities.  Cohorts who's player doesn't post frequently enough should be easy to NPC.
I do still think cohorts should be on a 25 point buy.  25 point buy characters can be every bit as fun and interesting as higher point-buy characters.  In fact, certain characters are more easily made with a lower point buy.  Others, of course, can only by made with a much higher point buy (such as Aragorn/Conan types).  Like Moorcock's Eternal Champion/Eternal Companion, a cohort can play an important and fun role, while knowling all along that he/she is no match for the PC he/she follows.