There are a lot of little bits in Wine that are missing before Java can run smoothly. (Yes, Virginia, we need win32 JVMs to run in Wine, for many reasons: some installers require them, some apps bundle them, and some people just plain want to run the win32 version of some apps because that's what they're used to.)
For instance, this bug says that we need to implement a little bit of IPv6 support to get Tomcat running: http://bugs.winehq.org/show_bug.cgi?id=7069
And http://bugs.winehq.org/show_bug.cgi?id=6380 says we have some bug in msvcrt that keeps some versions of Java from running.
This would be very useful indeed, because of the installers that require Java. - Dan