It isn't dung. A HC may not want the GM job but they still want a say and influence. If you have a guy you know you can do that with however you trust he is competent enough to handle the role, that is all you want as an HC.
I think a lot of people are misunderstanding the GM role. I think that the best GMs -- the guys who survive for 15-20 years in the role -- are great
managers. Even elite talent evaluators are slaves to draft positioning and available prospects. Successful teams inevitably have a brain drain of assistant coaches and front office staffs. Good GMs make good decisions about (1) who they want around them in terms of directors, scouts, etc., and (2) where and how to allocate resources on the roster.
This second factor is driven mainly by the vision for a team, not the next team. Sometimes a good GM will have a clear vision for what makes a good team and the head coach he selects will share that vision. I think it's better in the long run to have a head coach that has a clear vision and a GM who can start slotting talent to fit the vision.
I don't mind creative conflict between HC and GM, as long as the vision is shared.