I assume one spell can't have two different formula's because I have yet to see it, that I can recall. Given that, maybe it's restricted by mechanics. I don't know.

Saying WHM never pull hate is a bit of a stretch. Maybe it's just the way my LS does things that cause DD's enmity to decay through getting hit, but me and my friend pull hate on WHM and end up tanking some NMs for about a minute or 2 sometimes. That's at -50 ENM cap. The only reason we are pulling hate is because we do still use Cure IV. A WHM should never wait for a DD to take enough damage to use just V and VI.