For the same reasons any 64-bit application is desirable, to be able to address more than 4G of VM. Although in this case it's not a particular app but the JVM for Java apps so in theory a64-bit JVM ...

Back in the early 1.5 days, yeah, the JVMs were buggy for us. Since we've been using late 1.6 VMs from Sun -- it's been fine. No issues whatsoever.