This was a really interesting read, and it reminded me of discussions we've had at Lincoln Loop recently about whether to check requirements into the repository or install them separately (with pip/virtualenv or npm or bower or whatever). I've always been in the install separately and add to gitignore if needed camp, but I understand the opposing viewpoint. There are definitely pros and cons.
It was originally shared by Ian Bicking:
I heartily agree. It is a little awkward though, you find yourself discouraging packagers from doing work, and yet they keep insisting.
Also reminds me of the weird obsession with /usr/lib vs /usr/share.