At this point, I think that Ruby on Rails is bigger with people that are building things themselves than it is with people that are paying other people to build things. That may change over time.

Do you feel like you are being hired now because of your PHP skills or on your general reputation as a web dev agency? Most customers probably do not care.

Use what makes you most productive. In your shoes, I would certainly be giving RoR a good look as I like it better myself. If you find that RoR makes your job easier then use it. If not, maybe do not bother. There are many Rails inspired MVC frameworks for PHP though which you could look at as well.

There is certainly more buzz around Ruby on Rails these days so it may make you look more plugged in sometimes.

I do not see a problem with expecting devs to learn it. Your better devs would probably love the opportunity. With the world going Agile though, it probably makes sense to let the devs choose the technology unless there is a market-driven reason to change.