I feel backbone.js is a great base to build a custom mvc framework on top of it. However, for simple projects, I find it to be to heavyweight. I.e. it's really hard to have nested collections (You have to rebind all events). Views could give you a better hand for 90% of scenarios.. These are only 2 examples, but this is why I say it's more of a meta-framework. (I use backbone and patch it with lots of function, event binding, etc. and only then I can use it in my code).
So, I think there's still a place for a smaller, more 90%/10% mvc framework.. and have backbone.js to fallback on in the 10%. In fact, this framework could be build on top of backbone :)
