That's why there are quest notes and at least in Hydlaa an NPC who can tell you the location of any other NPC in this city.
I've only recently started using quest notes. They are indeed great, but if you forget to use them once you may get blocked (I'm prolly not the only one who has more than once
). An idea could be automatically logging quest dialogues inside quest notes, though I think that's a separate matter from marked NPCs. I didn't know about the location NPC however, although I remember coming across something like that once. That was a nice idea
But really. The only thing I believe these icons would do, is showing info on a NPC when standing near them without having to click them to see it. Pretty much an alternative to going to each town and double-clicking all NPC's one by one (which would prolly take half a day with how many there). Only to go back and do it all over again, because a NPC might have been intermediary in the quest
Of course if you have several forgotten quests, that you can only do at random.