For instance because you have componentized your product into different repositories, use different combinations of the components in different projects, but are still actively developing many of them simultaneously on each of those projects. Having to build, deploy and fetch gems (or whatever other method of distribution your language uses) is cumbersome if development is still really active.

I don't think you should ever want to bring unreliable external components, like a random github project that you aren't actively contributing to, into your tree like that.