Erlang is a standalone language but there is work being done on Erjang which is targeting the JVM. You're right that Scala and Clojure are targeting the JVM, and there are versions of Ruby and Python targeting the JVM as well (JRuby, Jython).
Yes, the JVM is a very mature platform and modern JVMs are able to optimize code and compile it on-demand to the host's native code for increased performance.
Yes, portability. Compiled Scala, Clojure, etc can be packaged using standard tools and distributed to any system that has a JVM (of a suitable version level).
But I think you've missed one of the biggest benefits of the JVM: the huge ecosystem of libraries - both the Java standard library and the vast array of third party libraries are accessible to any language that targets the JVM.