What if I need to know if partner has the Ace (or King) of trumps?
Maybe there are other/better ways of getting to this point, but what now? If partner has the ace of hearts, I want to bid a grand slam; if not, I want to stop at the small slam. Bidding 4NT won't help me (unless there is a way to continue cuebidding after a 5D response); bidding 4S will allow partner to show the ace of clubs but by then it's too late to bid blackwood.
Perhaps this is the reason exclusion blackwood was invented - but are there any more standard approaches to figuring this out?