I agree that they're worse than useless when trying to farm. If my mayor has a task like the thistles or knotweed I'll sometimes ask the adoptee to help clean up the town; if s/he has to catch things like the killer bees or slugs perhaps I'll get the adoptee swinging a net. In both circumstances they will perhaps get only one or two items at a maximum, though, so YMMV on whether it's worth the extra bother.