We shouldn't have bots in the rumble that can only be run on some clients. It's not really fair, and it makes us dependent on a few (or one) clients to get complete pairings for new bots.
There have been discussions about Scala in Robocode before and I think the current state is that we can't officially support it because it requires some insecure stuff disallowed by Robocode sandbox (like Reflection). See Talk:Other JVM Languages and this robocode-developers thread. Maybe we can revisit this discussion, but we'd need input from Pavel or Fnl, and possibly some Scala guru.
I'm also interested in Scala, and it might help us attract the edgy young functional programmers of the world if Robocode supported it. So it would be cool if we could. But I don't really know the details of how to do it right.