This 
… seriously, have a look at Design decisions you might disagree with and expand Quest markers are only shown, if you are close to your objective.
Also: disclaimer to avoid potential confusion
I get your point, but I guess implementing your idea is more demanding than it might seem. It would also be for a small subset of the audience that keeps replaying it, but cannot memorize quest objectives.
Actually I think there are two benefits, if you need to search a little longer:
- You gain more reputation and infamy… at least as long as there is still stuff to kill
- You might find secrets you have so far missed