Power's relative and if you're underpowered compared to your opponent then, sure, your Roshi may not steal you a game... but most ninjas tend to die fairly quickly when you're facing someone with much more power than yourself.

The one thing I'll say that people who pay have as a huge advantage with their roshis is that it's probably easier to have mood scrolls invested in all our ninjas and Roshi benefits greatly from being as heavily mooded as possible. An un-mooded Roshi isn't going to be super-impressive.