"When should a supervisor be a co-author?" One situation is when the paper wouldn't get published otherwise. For example, yesterday (literally) I submitted for publication a paper that I coauthored a paper with my Ph.D. student, which we contributed to equally. I asked him a few months ago if he was going to publish this section of his thesis, and he said he wasn't since he was too busy with his current job as a software engineer at Google (he left academia). I then suggested that we coauthor the paper, and I would do all the (substantial) work of getting it into shape for publication.