That's a great question and I think Jackie has hit the nail on the head. With a virtualized environment like this (either demonstration, training or testing), it is important to remember that end users who grab an image from the system to work with are ultimately going to be giving back that image at some point and these images will return back to a pristine state from which they came. So any changes, mistakes, additions or deletions to the virtual environment are going to be wiped clean. Thus, it is extremely important for whomever is building, maintaining and performing administration over these master images to be sure to keep them up to date as much as possible. That means keeping up the OS patching, application patching, etc... any normal maintenance that needs to be made should be done on some schedule to roll out new master images (keeping in mind that you may have child images attached to that parent image - so you don't want to destroy the parent file and thus lose the child images).
Adding virtualization to the mix makes things so much easier on the deployment side of things, but it is important to keep these things in perspective as well.
So good for you HiEdTechie for raising the question. It is a very similar thought process to creating ghost images in the old classroom environment days. After every class, if you roll back the desktop to a clean, pristine ghost image to remove any student information from the system, upgrading the host or applications on the machine with patches didn't make much sense. However, you still wanted to keep your master ghost image up-to-date, since you didn't want to reimage a machine with a year old OS image.
David