I've heard various ideas such as limiting it only after mods have expended their points (this will require implementing a cooldown to prevent a user from getting points again too soon). I want to hear your feedback, and I'll roll together something for the next major update of the site. Leave your comments
I think the main problem is that the selection for mod points is asynchronous (if I understand it right). That is, the user accesses the site, and gets no mod points yet. Then, while the user is reading, he gets mod points, but doesn't see them because he doesn't reload the site (there's no need to do so, because he's still busy reading the current version). He may even then quit reading for now. Then, at a later time, he again accesses SN, and gets a message that he has mod points. But they are to expire soon (because all the time they were sitting there without him knowing anything about that).
I think the right thing to do is to only tentatively give mod points in the asynchronous process, but really activate them only the next time he loads a page (so at that time — the first time he can learn about them — he gets the full period).