For scripting, that's definitely a problem. But it's not a big deal for long-running processes (e.g. a web application server). But Ruby definitely benefits from both: a fast Ruby with ~zero startup time, and a fast Ruby on the JVM.
3 years ago, Ruby implementation problems became the hot topic at Ruby conferences. Ruby was a beautiful language with an ugly implementation. It's great to see that the community has executed on this concern.
While I agree that it's not important for long-running applications, there is still a lot of scripting done in Ruby, and it's very frustrating to get a a 5-to-10 second wait just to tell someone their arguments were invalid. Waiting that long for tests is also such a drag on iteration.
I think once JRuby has stabilized the native gem support and has some sort of Passenger-like deployment option (Glassfish and Warbler isn't quite there yet), we'll see some big players built on MRI start to move their Web applications to the platform.
3 years ago, Ruby implementation problems became the hot topic at Ruby conferences. Ruby was a beautiful language with an ugly implementation. It's great to see that the community has executed on this concern.