It's overly complicated to test and release user scripts and gadgets if you want to develop something more complicated. If you use any build scripts you have to push changes manually.
Who would benefit
Gadgets and user scripts developers.
Allow hosting git/svn for scripts or integrate with Github (or similar service).
User script could then be a proxy to a certain file in the repository. For example I create a wiki page User:Nux/script.dev.js and write something like:
A release version is setup by an administrator by creating a Gadget pointing to something like:
Proxy would be used to both avoid cross-domain issues and cache scripts. Users would be able to use action=purge to download a fresh copy.
If git would be hosted on Wikimedia servers then you could add a nice auto-purge mechanism that would trigger purge when changes are pushed to the server.