Yes but, generally speaking, urban farmers know which crops are worth growing and which ones aren't. This is why you won't find many of them attempting to grow apple trees, and a whole lot of them growing tomato and basil. Except for the occassional case of an idiot urban farmer (or, more kindly, an overly ambitious), it makes no sense to criticize urban farming on the ground that not all potential crops can be grown better in one's back yard. (Except the relatively extreme view that all agriculture can be replaced with people growing crops in their back yard -- a view that I've never heard a person espouse.)