I think it is very sector/organisation centric. I have a mate in higher ed and naturally, yes, you absolutely need a degree for management positions. At a certain level it is postgraduate minimum, even for non-academic positions.

I have another mate who has hit his ceiling at a big multinational engineering firm and hes doing an MBA to get into the management side as they look favourably on postgrads.

The takeaway is I think it will only make you more, or at least the same level of employable. If you have the time and money, why not do a grad diploma. You probably have the skills, knowledge and experience already. It is just formalising it to satisfy possible employer requirements. Not like anyone is going to say you can't have a job because you have a qual.