I expect backbone forces you to follow good principles such as separation of concerns (data-model from UI code etc.)
However, it's still possible to follow good principles using your own design and JQuery, which means you're tied into one less framework and have more flexibility.
If you're developing with a small team, or on your own, then maybe you're disciplined enough to go down the more flexible JQuery route.
If you've got a larger team, then maybe you need the more rigorous constraints of a framework like backbone to make sure everyone is coding to a sensible structure, and you don't end up with 5 design ideas merged together in one big mess.
That's my thoughts anyway. Frameworks can offer essential constraints to help avoid a mess but they might be an unnecessary burden of constraints aren't required.
If I was doing a project on my own. I'd use JQuery and unshackle myself from a higher level framework. If I was working in a team, I'd favour the safety of a framework to prevent too many people doing things their own way.