I'm using dual monitors and wowhead. And yeah, I can agree to Republic - after some years of questing you know how Blizz builds the quests and how to do them or where to find some special mob.
Most of the time I'm using wowhead only to find out if a group-quest is easy soloable.
But TourGuide's also nice, because sometimes you just want to go "brain afk"
. But I'm only using it, if I didn't quest in the area with some other char before.
In my opinion, a lot of addon-help while leveling "kills" kinda the atmosphere.