I think there should not be a cap.

Example:
Yay! I just hit 25, now I can run the dungeon! Man, it is so effing fun .

..6 ranks later...

Boo.. I just hit 31(if 30 was cap).. now I need to wait 14 more ranks till I can do the r45 dungeon or any dungeon for that matter.

I see why people want a cap, I just feel that r31 - r44 people will be left in the cold while the rest of us are having fun.