And there's no way I'll vote up a poll with ten options including Dart but not JavaScript.
2) You can build things really fast with Meteor.
3) After you know what you're doing you can develop the robust version much more quickly. This time savings is often significant.
4) Since Meteor is built on Node, you don't throw everything away. You can actually keep quite a few functions and nearly all your handlebars intact when you port to the final project.
I'm not saying working this way is for everyone, but it works well for me.
It would help a lot in building the road map for an application, but you definitely wouldn't want to use any of it in production.
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.