This is also a reason that Cloud9 makes sense for us. The group I work with is focused on teaching programming to people who aren't traditionally represented in software development and sometimes they don't even have their own computers and have had to borrow a computer from a friend to do the work.
With Cloud9 they can develop on any computer they have access to, whether their's, a friend's, or even one at the public library. It is just one less barrier to being able to engage people.
I've run into the use case while traveling abroad where I don't have a computer (out of paranoia / lack of security).
Internet cafés are everywhere but setting up the perfect dev environment takes so long... so far it's made development fairly impractical, especially because they're often very old Windows machines. I've had good experiences with C9 so far, though I'd like to see how it pans out under that use case.
>setting up the perfect dev environment takes so long
It is a Not Unreasonable (TM) time investment to automate setting up your dev environment in the way you would a prod environment. emacs.d and other dotfiles in source control, masterless Puppet or Chef on your laptop, etc. If you're willing to go as far as installing Vagrant manually, the rest can be made pretty easy.
With Cloud9 they can develop on any computer they have access to, whether their's, a friend's, or even one at the public library. It is just one less barrier to being able to engage people.