It may be true that the worker handling self-serve checkout is worked pretty hard sometimes fixing things and assisting people checking out, but the typical arrangement around here is that they have one person monitoring and assisting 6 "20 item or less" or 4 unlimited item self-serve checkout lines, and they mostly seem to manage to get the job done.  At the Meijer i get groceries at, they have one or two workers, depending on how busy it is, overseeing all 8 self serve lines.  Maybe the person doing this duty is paid a little more than a checkout clerk, not sure.  At any rate, the stores are saving quite a bit of labor in the bargain, seems like to me.