If you do all of the dungeons from 1-50 you only need to do few repeats in the same dungeon. If these hardmode dungeons are there to add variety to 40-50 grinding range then I'm not sure we even need it.

I second the idea of making them alternatives to end-game grind. When you get to 50 the gear progression forces you on a bit too linear path.
Grinding in this game is so fast that I'm pretty sure I'll get each class to 50 at some point.