It would be great to be able to specify which components should be checked out (during pipeline).

One application for this is the checkout of a "pipeline configuration file" which is stored in the stream which should be build. This way a generic pipeline can be implemented which is then configured by a configuration file in the stream which is built. This generic pipeline (which is stored in a separate stream) can be used to build multiple streams.

It is bad you have to check out all components in the workspace/stream if you only need one file in a single (small) component.