I've quit several times although I wouldn't call it "quitting" rather a rearrangement of my priorities. When I get my life priorities in order I usually come back. This is normally caused by college and making sure I keep my grades up etc.
If you can manage your priorities and put other things ahead of WoW you can play without dedicating too much time. Although raiding is a ginormous time sink I am not sure how these folks manage keeping everything in their life healthy.