They did in 1.23 when they made DRG a DRK. (Its why they were so scared to try again, since they botched it)
But in all seriousness, they also had the perfect set up for implementation back in 3.0 when DRKs MP was being constantly drained. All they had to do was make MP drain before HP, and at smaller amounts. Once out of MP, it takes HP, and is less potent (to limit DRKs to only trying to burst at the last 1% of a boss, or the end of another tight DPS check)
(I also agree to putting short CDs on HP spending skills)
