Read-ony makes sense, and something like Cloud9 makes sense. But just editing a few files? You might have to help me out here. Genuinely requesting.
This is a much easier way to let developers evaluate a project without having to go through that hassle.
This is an area where right now most projects seem to rely on (the awesome) jsFiddle - but having things on your own server makes sense in many cases.
Because it's REALLY awesome, if you're having problem making something work in a browser, to be able to copy just the relevant conditions that break into an editor like this that you can share with the world. I wondered for a long time why people used gists, or sites like jsfiddle, but the first time I saw somebody post broken code to a fiddle (via StackOverflow), then saw somebody else fork it, make corrections, and post the corrected fiddle back, I immediately realized that whomever had thought of it first was a much wiser man than myself.