It's really hard to approach this kind of topic and respond to every reply due to dungeons lack of place in the endgame primarily. The importance of dungeons is currently in the eye of the beholder because SE hasn't given us a definite use for them, On one hand someone is correct in saying dungeons are essential to helping people learn their class during leveling however that shouldn't be taken into consideration at this stage in dev.
Apparently there's also people here that think they're actually rewarding......
Variety is another thing that you just can't provide a definite response for, one might think there is enough different ways to cap out tomes and others may not. For me I don't think variety is a good enough reason to have something, its a good thing but not a real purpose.