I fail to see the difference between a supervisor and anyone else. To decide whether a supervisor should be a co-author, you should apply the same rules you would have applied in any other case.
I generally prefer to be inclusive and if someone was involved in the project, then I think they should be offered to be co-author (unless it is obvious that their contribution is very minor). It is up to them to decide whether to accept or reject the offer. If you do not like their answer, next time do not talk to them about your work.
Also we are moving to a world where academic work in measured and rewarded very precisely by criterions criteria which are very narrow. For example, in the UK the REF, which measure the performance of departments, in the individual level looks mainly on the quality of papers. So there is little incentive to individuals to supervise students (department as a whole do have some incentive). Now, in most cases supervisors put a lot of work into the supervision. So morally and to encourage people to keep supervising students I see no justification not to give supervisors credit when they deserve it.
