The limitation is because data motion uses synchronous snapmirror as the underlying mechanism to synchronise the volumes in the final steps of the cutover, and this is one of the limitations of synch snapmirror. Without a large re-write of the way synchronous snapmirror works this is unlikely to change, and a large rewrite of synchronous snapmirror is probably a few years away.
Having said that, there are other ways of doing what your looking for (Non disruptive volume migration - coming soon), which will allow you to move volumes between aggregates on the same controller, and the good old "snapmover" for moving all the vfilers on a given aggregate from one controller to the other (basically the whole aggregate moves). With a little luck this could all be included in something like provisioning manager, or failing that a relatively simple powershell script.
