If you've been told the exact location of the place you're supposed to go, there's no reason why it shouldn't be marked on the map. It really doesn't ruin exploration more than when you're told to where to go. Still, it should only show you the entrance, and places you supposed to know, not the exact position of the item you seek, except maybe a wandering NPC as they can be annoying to find...
Exactly my thoughts. Quest markers aren't a bad idea (imo), but in Oblivion they were overdone, showing you the exact location of everything no matter whether you could know about it or not. In Morrowind on the other hand it was extremely annoying to search for a dungeon entrance for half an hour sometimes. Didn't feel like exploring, it felt like a chore.