They are good for casual players and first time t.e.s players who have come from other rpgs with markers like final fantasy or f.p.s games with similar markers.
Another thing I can imagine why they are a useful inclusion in your games is to limit the traffic flow of the forums, a lot less "Where is x" and "How do I find y"
So I understand they must be in the game. But for some guys like me, they make your games less of the adventure and exploration and journey that Morrowind was, and more of a train ride, watching the scenery as I march from a to b 100% certain of my destination etc.
So all I'm saying is it would be really great if we could simply switch quest markers on or off. Then people can decide for themselves whether or not to have them; A simple choice that I think affects gameplay immensely.