I agree, this was written from the point of view of a miner estimating his expected gain from one specific block (or more precisely, one round), but as long as blocks (or pools) are exchangable, there is no reason to do this. If he has a choice, a small miner should switch to a pool with a "younger" round. I don't belive he should start mining all by himself is there is no other pool, though; his expected gain would drop close to zero, because of the increasing difficulty.