I stickied this topic. Allowed deps are as of present gtk2, python, perl and things like that. There was a discussion on making every scm completely self contained relying on nothing else, though that is a huge overhead in the amount of effort to create each package.
I think the main use of the scm is the fact it is simpler and less breakable than the standard setup.
But with TC 5.x around the corner, I have been spending time in other areas besides scm. Sending in scm extensions is always welcome of course, but my own activity has been paused.