The Middle Distance

It doesn't happen very often, that's for sure.

It's never terribly clear in canon just how involved they're supposed to get either. :) My thinking is that he's supposed to encourage humans to be at their best but not directly get involved himself unless told to do so. Divine intervention should be used sparingly, after all. Can't let the little buggers get too used to it. He and Crowley can take each other on directly because they're counterparts. Both can and do use human agents to achieve their ends. In this case he's suspecting it's part of the Plan for this monastery to not last forever. There's little he can do about that.

Further complicating the matter is that they both break their own rules all the time. It's telling that Aziraphale is most tempted to do so to defend the books.