It's still great that JRuby beats plain old Ruby in speed and also gives you Java access. The only (relatively old) benchmarks I've seen with YARV are over here:
http://antoniocangiano.com/category/ruby/
... and for me, it matters more if Rails will run faster on jRuby. The benchmarks may or may not reflect an increased performance advantage for a Rails app.
Or at the very least, be able to compile a Rails app into bytecode for commercial distribution. I would have a use for that even if jRuby doesn't run as fast as MRI.