Right, but the higher the percentage of pool hoppers, the higher the percentage of the time the pool's expected payout per share is below average. This means that the more pool hoppers, the lower the pool's payout to someone who contributes shares at a constant rate. That is, pool hoppers profit at the expense of the non-hoppers.
As I said, that is not likely. If a bunch of pool hoopers have invested shares in a small pool in the begining of a block (they seem to do that until some point early in the block) and this pool is currently starving (left with a huge block to solve and little hashing power to do it), pool hoppers will go back to it helping finish it (as long as there's no more profitable alternatives).
And it's not accurate that "pool hoppers profit at the expense of the non-hoppers". After all, they have actually invested hashing power... they just point their hashing power to some (currently) more profitable pool... IMHO, it's the same as investing money in a more profitable stock.