[tempestn]

The proposed change would allow any GHash member to essentially steal mining profits from GHash (by taking the full reward for any blocks found, while still sharing in the reward for blocks found by other members). Since some participants will certainly selfishly take advantage of this opportunity, there would be a disincentive for honest members to belong to the pool, and it would therefore quickly lose its dominance.

Edit: And more generally, for the same reason, pools of this sort should cease to exist.