Besides, small pools might benefit from pool hoppers in the first place, since, at least for the time they hop into it, they might have more mining power than they would ever get otherwise.
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.