Is it just me or does Death's Due seems really not finished and poorly thought out?
I'd put off making a discussion about this until DKs get their 'big changes' pass (here's hoping that still happens) but with blizzard advertising the covenant abilities today and the announcement of beta soon the spell is unlikely to see significant change.
First of all, the spell is just wonky (and blizzard has been leery about making it available for testing) for the same reason defile is wonky. If you're playing any content but a patchwerk raid boss 'properly' you are unlikely to be standing still long enough to get the full benefit out of it.
Secondly, Is this the reason Frost had DnD readded to the spec? Because it's a dnd modifier and frost has no dnd synergy like blood and unholy, frost also mechanically benefits from this spell less than the other two specs.