It seems like the problem might be that people assume agile software development is, in and of itself, supposed to be a methodology. As I understand it, agile software development is meant to be a set of principles describing any methodology that serves those principles. As such, Extreme Programming is an agile methodology, intended to serve those principles when employed well, and Scrum is a pseudo-agile methodology that serves as the halfway point to which one might be able to drag management, but more often ends up being an excuse to measure developer productivity in story points.

There's value in enumerating important principles for consistently pursuing quality work product. That's why the Agile Manifesto has value. The manifesto is just a manifesto, a statement of intent and principle, and not a methodology in and of itself. One doesn't need a published, monetized methodology to engage in agile software development. One simply needs a methodology -- whether unique to one's project, or widely known -- that reinforces those principles to engage in agile software development.

The biggest problem with agile software development as a concept is that it, like any other elaboration of any principles of quality work, is that most people will fail to see the forest for the trees, will marry themselves to some well-marketed thing that pretends to be about principles, and will end up largely doing the same dumb crap they've always done.