I would say that your problems were not caused by a long-running branch per se, but rather were caused by the intern not pulling from production often enough. Linus talked about this in one of his Git talks. Responsibility for merging should lie with the team working on the new feature in the side branch, not some other team that knows nothing about it.
Agree (the intern was hypothetical, but agree that long-lived branches are usually symptoms of human rather than tooling problems). However these things happen, and if you are in a hole, it's nice if your tools let you dig yourself out :)
