Quest markers aren't generally a dumbing down thing. If you use them to mark a general area to avoid wasting time on an otherwise yuge map, it's a good thing.
If you do it Bethesda style, where a quest says: "can you find the fabled treasure nobody found in 1000 years?", then mark it on the map so you just need to go there and pick it up, that's horrible.
It's all about balance.
With quest markers you don't have to be able to read and you don't even have to be able to listen. There's less and less need to pay any attention whatsoever.