I agree strongly. I even have your old code for this sitting around. The issue is that a plugin expecting the vm to supply session id would have to run with a vm able to so do. It seems some people mix releases and might get upset by this. Personally I say to hell with them. Others might disagree.
Backwards compatibility is important but I think we can dodge this one easily. Because the worst thing that may happen is that you use a FilePlugin which does not use the VM's session ID but its own one (which may not be nice but is certainly not fatal). The other way around (a newer file plugin working with an old VM) is easily prevented by increasing the vm proxy's minor version which we need to do anyway if we extend the interface. In short, I don't see an issue with backwards compatibility - old versions will continue to function with new VMs and that new plugins may not function with old VMs is a fact of life.
Cheers, - Andreas