I don't see it replacing RubyGems.org or npmjs.com anytime soon. It could be useful for some open source projects (maybe as a beta/prerelease repository?) but since it's scoped to a single user/organization I think it will be more useful for private packages.
I surely see a nice use case for it here at Mikamai, where I work, since we have some private gems that we cannot release as open source for legal reasons, and we have to download them via Git using access tokens. Being able to use our GitHub accounts with the same ACL as our GitHub org would be wonderful!
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