Co-founded Nitrous (but no longer part of it) so take my opinion for what it's worth. The real benefit is not in the Web IDE per se, but the fact that your dev environment is accessible from anywhere. That is the killer feature. You don't have to setup your environment more than once if you have more than one computer; you can work even with a Chromebook/iPad, and it's easily shareable with your team. The web IDE is…
- When doing client work it's much easier to show clients what you're working on at any time. Don't need to mess around with a staging server and uploading files + DB changes etc.
- All client projects are in their own environment with their own config / env vars / DB. So say a client wants changes made 3 months down the line it's super easy to get the project back in the state it was then. On a local machine I frequently run into problems where I've updated my machine or changed config since last working with them and it takes an hour or two just to get the project going again.
- When learning new languages can just create a new project of that type and it has everything you need ready to go. Especially useful if you have a windows machine where using NodeJS / Rails is often much more painful than it should be.