I work with a person who drives me crazy (okay more than one), but this one person in particular I used to think was absolutely stupid because she couldn't do even the simplest of tasks. It was often quicker just to do them myself than to try and teach her. That's when I realized she was actually brilliant. I still think she is dumb, but not as dumb as she pretends to be just so people will do her job for her.
Some people will think she is smart, but eventually, they run out of people to do the work for them and are left stranded. I know a person who does something similar with friends, only making and keeping them while he can get something out of them. He is quite lonely now I believe, and not even his family are in contact.
Our neighbor is like that too. This lady used to physically cut and paste stuff for documents.