I in general dislike throwing up barriers that prevent objects from being dropped. As a user, I find such rules frustrating, especially if I'm still allowed to accomplish the same drop indirectly by some series of commands (e.g. REVOKE first, then DROP). If I'm allowed to do it indirectly, then I should also be allowed to it directly, at least in cases where there's only one way of fixing the problem that is preventing me from doing the DROP, which I think is the case here. For example, if bob owns a tractor and I want to DROP bob but the tractor is indestructible, then it's reasonable to make the operation fail. I need to give away bob's tractor before I drop him. But here, if I say I want to DROP ROLE b, I'm going to have to first REVOKE c FROM b -- there is no real other alternative. So why not make that happen automatically? When I say I want to DROP something, I'm serious: I really want it gone.
I'd rather that intent be communicated through CASCADE than just assumed, but I agree with the general point.
