When you have programmers who are more proficient in writing JavaScript than Ruby or PHP. Better a PHP expert using PHP than a cowboy coder using Node just because it is the new hotness.

Assuming this framework is as mature and full featured as Rails (which I doubt), then hell yes this would be awesome compared to using Rails. Also you have to assume you (and other devs) are equally proficient in both languages.

The reasons:

- DRY! Use the same templates/models/controllers/i18n/validations on server and client! I can't underestimate how awesome that would be.

- One fewer languages to learn and switch back and forth between

- Team flexiblity. No such things as "front end guys" or "back end guys" (sorry girls)

- MASSIVE improvement in debugging. Debugging in Ruby is just bizarrely crappy. Really WTF? How is this possible. Anyway it sucks badly but is pretty decent in JS. (assuming stack traces were decent, this is easy to screw up in js)

Plus there are the inherent benefits of using node (arguable) where you could do push notifications/websockets/etc trivially and in a well integrated way instead of having to... use node (or faye/whatever) or running some other server.

Some people might argue that js/coffee script suck compared to Ruby and cite that as an advantage but I for one love js (and ruby but less so) so I don't care.