Would it not make a lot more sense to load a users' global scripts from its home wiki, as designated in Special:CentralAuth? I am rarely on meta, so requiring users to store their global code there seems overly complicated.
Topic on Extension talk:GlobalCssJs
Q: Why not put global css/js on the user's home wiki?
A: Which wiki is considered a user's "home" wiki is essentially random, cannot be changed by the user, and could be someplace with much laxer requirements for becoming an administrator than meta.wikimedia.org, like test.wikipedia.org.
Nope. Or at least this is something I wouldn't be able to see as an improvement until CentralAuth has a way to set your home wiki. MediaWiki.org is my home wiki in terms of edits and user rights and overall activity, but CA thinks my home wiki is test.wikipedia.org, where I have a whopping nine (!) edits and some user rights. So no, I wouldn't want to have my global CSS & JS stored on test.wikipedia.org; Meta in the context of WMF wikis makes total sense to me as it's a central place for coordination.
Yes, what Jack said. CentralAuth has some unreliable criteria with which it picks a home wiki, and it's really only used to figure out who wins the account. Also, from a security perspective it is much safer to only allow meta admins to edit your global scripts, as opposed to any testwiki admin :/.