@atomicpoet @maegul to add to what you said, a flexible protocol allows for new ideas to be implemented, ideas that weren't even considered when the spec was defined.
I'm sure the Diaspora protocol is great for implementing Diaspora. But people experimenting in this don't want to implement Diaspora. They want to build something completely new. AP provides the underlying framework to allow these applications to link to an existing network. It lets them take advantage of the network effect as it were.
Most applications using AP are still microblogging platforms in one way or the other. The idea to tie a code forge (was it gitea?) platform to AP is one of the more interesting ideas, and something you could never do with Diaspora, I think.