I suppose it depends on the disability.

I don't think it would be particularlly good to have a person with multple personallities or schizophrenia working in retail. Great way to scare off customers. Mild MI, sure, no problem. A depressed person woun't scare people like a person talking to his hallucinations, or disrupt things like a person who one day thinks they're "Susan Smith" and then gets stressed and thinks they're "Mary Worth", or worse yet "Aeris". Not to make light of it, but you see the point, especially if one of the personalities is violent or something.

Same with a disability. If the guy is unable to do things and keep up, he shouldn't be doing them.