[BjoernKW]

If you're developing a library dual-licensing is a reasonable option.

Many open source developers probably shy away from that option though because they're afraid that fewer companies would use their library in the first place and they're also probably not entirely mistaken:

Enterprise companies are wary of effects licences like AGPL might have on their ability to keep their source code private. Unfortunately, those companies more often than not aren't exactly eager to pay for a licence that allows them to do just that either.