Heh heh ... seriously, Illysia's "Blessed with suck" phrase comes to mind when I hear about these these quest depenent NPCs going M.I.A. Its as if whoever implemented it thought it would be wonderful for them to roam about, but forgot what its like to be a player who needs to complete a quest, or a confused noob who has no clue where any of these NPCs are to begin with.
The wandering NPC does add to the realism, but at a heavy price as far as game play and frustration level goes.
If there was a way to track down the wandering NPCs, then perhaps it would not be so bad. ( ie. if you could /shout their name in the NPC tab (please, not in main!) and get a response or something )