I can think of one, even though I sympathize w/the basic approach. If you participate in the primary and lose, there is some obligation to support the winner in the general election. If you don't, there is no obligation on the part of others to support you. With no such agreement as a routine condition, you end up with a split-level election. The primary becomes the first round, and the general the second. Not the worst thing in the world, but more taxing for the party so situated.
mbs