My main concern is immediacy. There's 0 setup time, and you can instantly share your work. Especially for (relatively) novice programmers, being able to immediately link to something someone more pro can see is invaluable.
I deliberately left dabblet out because command + shift + left (the shortcut programmed into my brain for 'select this entire line') closes the editor. Annoying as hell.
This post is talking about prototypes from a usability perspective, not a technical perspective. A prototype by your definition ( a "real" prototype) would use data and be built in about the same way as the final product would be. I'm talking about how to prototype the experience. I see how it's easy for you to misunderstand me - apologies for not being clear.
That's accurate, and I think that's all the more reason to jump in and start. The content is all there, but there's more detail needed to make an app like this awesome, including stats and UX polish. I'm advocating for the 'just start' approach.