Some frameworks are designed to help with this step though. For instance: Rails has scaffolds & generators, angular.js has directives (plain old html views you

Some frameworks are designed to help with this step though. For instance: Rails has scaffolds & generators, angular.js has directives (plain old html views you can open), html itself can be used for mock-ups (and there are plenty of others as well). Specifically, why build a partially functional prototype only to throw it away for another framework down the road. It seems naive to me, because the effort you put into the prototype is mostly dropped (except for high-level mental models). Rather then building a throw-away prototype, I think it's better idea to build a prototype you can refactor and improve, with one set of tools.