If everyone did, the share difficulty would be through the roof. 2,222,770,798 if I understand it right..
p2pool is not the answer.
a rewrite perhaps. but not in its current incarnation.
I was thinking about this the other day, actually, and I think I got a good solution to the problem. You can read it here:
https://bitcointalk.org/index.php?topic=18313.msg10519027#msg10519027