I guess it's neat that they "self destruct"
But how is this much different than setting a user's AD account to expire and just reimaging a machine with a customized image? I guess it could be quicker with the VM, if the VM template is already stored on the local disk. But if not, I can't imagine it really helps you too much?
Plus, what about data? If there is data stored on the local machine, say, in the user's profile (and users do love to put gigs of stuff in folders on their desktops), does it too get destroyed when the VM expires? Some stored data, even for a short-term temp, is invaluable to the business or depending on the circumstances of the person's departure, may be needed for a forensic investigation..