"Defect against cooperate bot" makes sense if the Prisoner's Dilemma game is the only thing that's going on. However, in the real world, cooperatebot might have friends who will take revenge, or CB might be doing useful work.

From memory: a person who refused to defect in a PD because they didn't want to be that sort of person.

Defecting against CB is equivalent to "Never give a sucker an even break", and it might lead to a world where people spend a lot more resources than otherwise necessary on defending themselves.