Well it's all about sharing. Where there is a need, there is a PE. After that, it's about if you want to share it or not. Previously, heavy customization of e.g. Seam (beyond overriding built-ins) has indicated custom Seam versions but now contributions can take the form of PE:s, interceptors, decorators etc.
it seems like a good idea...
it's like toyota selling (or providing for free!) the engine and you add the brake system, transmission, etc.