I think I'm "rarely".
We started doing it for OEF repos last year. For example, https://github.com/Open-Earth-Foundation/CityCatalyst includes a Web app, a global data API, and the data pipeline scripts for managing that global data.
It's helpful for getting Open Source contributors up and running quickly. I think we could probably get the benefit of per-project repositories, but still make it easy to check out the full system, using `git subtree` or something similar.
Anyway, an interesting discussion.