But if you constantly switch guard nodes then chances are that very soon you will use at the same time two compromised nodes, probably revealing which sites you visit. Also Tor users usually use Tor to visit specific sites, not generally browse the web, so even if someone peeks into their circuits once, he can get a good idea of what the user uses Tor for. So fixing the guard nodes means that either no one peeks into your circuits or that someone peeks constantly into your circuits (well 1/3rd of them) for a long time, which is not really worse than peeking for a day into them.
I don't follow tor-talk.
I wouldn't mind of course having the option of excluding your node from the guard position.