I just kept running out of non-collection quests. (especialy at around 30-40) so It's handy to have instances to fall back on.

If you're doing an instance at a fairly rapid clip, the xp\hr can be as good as questing, especially if the level appropriate quests are spread out over several zones. Additionally the gold can be very good, my 5 epic mounts were partially subsidized by about 600 stacks of wool inflated to 3 gold a stack by careful market management.