It's a tricky subject. As you know I've put together a couple of add-ons. I suspect it is highly unlikely that anyone would develop an add-on without examining the Alibre dll. As I understand, the alibrex dll is the 'public' interface everything else being 'private'. That said, I've certainly referenced classes in some of these dll because there was no option if I wanted a working add-on. I have however always been careful not to distribute any Alibre dll with my code since that would be a clear breech of the license conditions.
I've always worked on the basis that if someone wants to use one of my add-ons, they have already got legitimate access to Alibre.