That is the direction that clckwrks has naturally ended up heading in. In the first version, it had blog stuff builtin. But as things got refactored the blog/cms stuff just became another general purpose plugin -- not part of the core.
In fact, the authentication layer is even taking a step in that direction. It is still hardwired into the core, but it uses the clckwrks plugin architecture.
At its core, clckwrks is just a general framework with allows you to dynamically load and unload components and themes. But it does not put many constraints on what those plugins need to do. I have taken existing standalone web apps and turned them into clckwrks plugins with little effort.
In theory, you can save development time though by building a web application that builds on top of existing plugins -- such as a media plugin, cms plugin, payment plugin, etc. That way you can focus on the unique aspects of your site instead of wasting a lot of time on the boring mechanics of dealing with payment processing, serving files from S3, etc.
- jeremy