Loryen
In my opinion, in a real sandbox, players can do anything NPCs can, almost (if not entirely) removing this reliance.
This is a strong driving force in the majority of suggestions I have made for this game, and the direction I'll push for any new suggestions that I can come up with. Well, that, and finding things which Iala doesn't like.
Mentoring doesn't really add much in terms of functionality. It trades convenience of staying in the field with a little extra time and experience spent "gaining" the ability you can just TP back to a capital city and learn. I would also be perfectly fine with teaching new abilities this way. I didn't suggest it because I figured more people would object to it that way.