Dependency control it is a solved problem. Which is why SVN has externals, Hg has hgsubs, and Git has submodules and now subtrees. This isn't a git-in-the-large problem, it's an almost-any-nontrivial-project problem. It only seems unnecessary 'till you've had it and tried to live without it.

Bringing other libraries into my project is beneficial because then my build system is able to wrangle those just as easily as my code. (And it's also simply useful in the case where I've written both modules but maintain a separation for whatever reason--one is an open-source project and the other is not, whatever--to be able to make changes to one from within the other, run the tests for the submodule, and push it up to staging or upstream, without having to leave my current project.)