What this approach doesn't solve, as far as I can tell, is the problem of eventually merging together two or more long-lived feature/dev branches. Sure, you know you have no conflicts with mainline because you continuously integrate that back into your work. But what about everyone else's branch?

The point, I thought, of Jez Humble's definition of CI is that you are continuously integrating /everyone/ and /everything/ specifically to prevent the above problem.