In a nutshell, a realm-based mentoring system can have great rp and narrative potential, especially because I find it pretty fitting the idea that one student is taught while embedded in a determined social environment given by a certain realm - as long as the mentor is also good enough to teach him game mechanics objectively at the same time. Clearly such approach would make the mentoring less effective as a newcomer picking the wrong realm (and he has really no way to know it beforehand) would risk to be lost to himself.
An island-wide or game-wide system (I doubt the latter would be appropriated or implementable though) would make mentoring probably more effective but clearly more OOC oriented.
In a sense it's sheer effectiveness (if island-wide) vs flavour (if realm-wide).
A related question is how important the absence of a mentor is in the decision of a player to abandon the game.
