There are numerous benefits to using a presenter, or at least a "renderContext" object. Most conversion, formatting, predicates, and lazy attributes won't come through with toJSON(). The performance penalty is negligible compared to the benefit of testable presenter methods, logic out of the view, etc.
I agree that putting all that logic and template vars into a template is confusing. Hard to create mocks of what the HTML should actually be structured like. Im using Jade so I can be terse and pre-compiling the templates. While in my Backbone application, I apply DOM-like rendering with Plates.js.
It's supposed to return an object that is suitable for turning into JSON, but it won't convert any of items members. JSON.stringify relies on the toJSON functions of the members to serialize themselves.
Performance isn't the only thing to consider.