Actually, each client could be given a different coinbase address. There would still be the problem that the individual client could run off with the entire block reward, but it would reduce the exposure to just the single client.

Furthermore, each client could be required by the pool to put up 50 BTC earnest money that the client would forfeit to the rest of the pool members if they failed to share the reward of a solved block with the rest of the pool.

This would allow each client to continue signing and hashing without needing to bother the pools servers, and would provide an incentive for each client to share the block rewards appropriately.

Therefore, since this doesn't seem to prevent pools, there doesn't seem to be much benefit to making such a disruptive change to the protocol.