GitHub running off the main branch is fascinating, and initially sounds mad, but makes so much sense. Assuming they have very high test coverage, running against mainline Rails isn't really any different to having the fork they had before, but they have more influence on future development. It must also be a massive boon for the Rails ecosystem to have such a large property running off the head. Doesn't anyone know o…
That's more because of our engineering culture. The cost of NOT upgrading outweighs by a huge margin than keeping building on top.
And yes, have seen Django shops locked into 0.9x release patched right into the core and running for a very long time, impossible to upgrade and all the horror stories.
EDIT: Added Django