In David Black's book, 'Ruby for Rails', he mentions that a solid grounding in Ruby can serve you, as a Rails developer, in four ways:

1. By helping you know what the code in your application (including Rails boilerplate code) is doing
2. By helping you do more in, and with, your Rails applications than you can if you limit yourself to the readily available Rails idioms and techniques (as powerful as those are)
3. By allowing you to familiarize yourself with the Rails source code, which in turn enables you to participate in discussions about Rails and perhaps even submit bug reports and code patches
4. By giving you a powerful tool for administrative and organization tasks (for example, legacy code conversion) connected with your application