I don't have a reason for switching away from Ruby at this point in my career. Things can always change down the line. Maintaining multiple languages at the same layer sounds like a personal nightmare to me. I'd rather learn to write PHP and have that be the only language, than to write Ruby+another language on the server-side.

We're building our company on Rails because we think it's the best choice for a young company, since it allows us to respond to customer feedback more quickly. That's what we're optimizing for right now!