I think a big part of the seductiveness of polar extremes is you know where you are. If you're an anal retentive perfectionist... and you start slacking off. You know you can say to yourself, "Self? Get the fuck back to work!"
If you slide off from that at all, then where are you? If you start slacking... how much is too much? What is "a little" time off? The thing with perfection is it's something to strive for, something to mark yourself against. How can you strive to be "mostly perfect but happier?" Not to say that you can't; it's just far more difficult if you're at all "goal oriented".
For me, it's really not so much being a perfectionist so much as I'm a workaholic. I like to show off. If I'm not doing anything then I don't have anything to show. Of course, that's not the whole of it. There's also the "need to complete" something that I've started... and ... oh, I don't know. My brain is frazzling and I'm worrying about stuff again. And I had another thought I wanted to think through in a journal entry. So I suppose I'll move on to that...