If I split our profits as a whole pool, proportional to shares submitted, and not for each coin individually, there are ways people can cheat.
Of course one cheat would be to detect the when you switch to a "hard" coin that has lower shares per time and go elsewhere until it switches to an "easy" coin. Then come back full force. Then get paid off per share at the same rate as the guys that stayed the whole time.
You could come with tricky schemes that try to punish people who do that but they also can ding the "honest" people that just have to drop off line for a while or whatever. Honestly, one of the things I like about Middlecoin is that it doesn't need that bs.