I don't know about the vamp, but the crusader is just about the only guy who is reliably immune to curse. Since he's also immune to poison and mana burn this makes him a beter resist stacker in some way than the guys who stat out with resists. He can also lug patches around with no downsides except random "ur dead" teleports.
I'm not sure whether this was intended or not. The combination of "no relevant debufs" + "no downside patches" + "extra altar to mooch resists out of" makes him very single mindedly powerfull - but has no sinergy wih the first ability at all
And curse is really dependent on not being easily ignored, it's unique as a staus effect in that regard. Crusader makes fun of that concept.
EDIT: Avatar beat me to it.