I guess I will add my 2 cents here.
I think we need to make clear the difference between the definition of role as a [b]function[/b] and as a [b]role- play personality[/b](I cannot think of a better term right now).
Some RPCs have roles that are very closely associated with their role-play personality, such as RJ, whose function as the drachorn 'manager' is closely associated to his role-play as the Drachorn Master. Thus, his disappearance could bring trouble as there is no drachorn manager around, but still, the only Drachorn Master is him, and to the same sense that MD tries to make an avatar [b]unique to each players[/b], it just doesn't seem right to give away RJ's role to anyone else. The function, however, can be given to new people, but I much prefer that they have their own role to play, not a Drachorn Master.. at least, a different title and a very different background.
Some other roles, is just unique to the players, and should never be transferred away, an easy example would be Metal Bunny. Do you want someone to be the new Lord of Bunnies if he's gone? I don't think so. Bootes, BlackThorn, Thanasia, Alche, are just a few amongst the many examples I can give for roles that should never be transferred to anyone else. Even simply's role as the ferryman, and aqune's role as the storm maiden (she is PWR, but with a spell unique to her) should never be passed on. Their spell and abilities(teleportation, weather spells), can be given to others, but the title and role itself should never be passed on. This is actually what has been done even now, with Morpheus's ability being shared with others (I know at least 2 others, excluding Mur, who has dream ability).
So to conclude, I think the roles that need to be periodically reviewed and be passed on is very limited to a few functions (drachorn master, spell pages, assassin, Wodin, and maybe KC), but to think of it, there isn't even that much roles that need to be passed on. It would be such a waste if the programmings done to accommodate some abilities given to RPCs/PWRs disappear when they retire, but I personally think that most of the roles itself does not need, and should not, be passed on, but only the few specific functions instead.