I think this is a great idea, if nothing else it is really the only way to effectively stop (or severely discourage) a block withholding attack.  

Say the person who finds the block gets 50% of the block, everyone else splits the other 50%.  Well, if you perform a block withholding attack, you would hurt the pool, but you would hurt yourself as well as you would get rewarded for 50% of your shares and no reward for blocks (as you withhold them).  This hurts the pool, but hurts the withholder a great deal as well.  Under the current system, any block withholder gets 100% for their shares (more or less, depending on luck) and has no contribution to the pool.  Perfect for a block withholder to come in and mess things up.

I'm not saying change the whole pool, but maybe a new pool with a 50/50 option would be nice.  A sort of half pooled mining/half solo mining.  It might be the best of all worlds - the steady income from the pooled mining with the security of solo mining (people would be crazy to block withhold and solo mine).

Personally, I think this is the best way to deal with the biggest threat to the pool.  Anything that makes the pool more secure and helps to guarantee luck is a good thing in my book.