I think he's arguing that Backbone.js in itself is fine but since it doesn't do much you have to add plugins or write your own implementation of the missing par

I think he's arguing that Backbone.js in itself is fine but since it doesn't do much you have to add plugins or write your own implementation of the missing parts. The issue with that is that the learning curve is now higher because the chances that a developer knows those missing parts that you added or coded is increasingly small. Btw, I'm a heavy Backbone.js user so I still think it's a better option compared to other MVC flavours but I do have the problem that a lot of things related to data binding and UI animations is custom built and have the issue described above.