One approach that makes sense in an enterprisy way is to use an ESB. BPEL can help here. Your ruby app calls the ESB on a service. Behind the scenes the ESB composes the calls efficiently into a final product for you. The enterprise system bus will aggregate the parts into a useable whole defined by your domain.

A benefit of this is that people get a unified view of the system. You can pull parts as needed. You can survive partial outages better than a monolithic app.

Of course you can still call each of the services independently. The ESB is just another tool in your box.