You see quite a common thread in a lot of communities where folks are just trying to to wrap their minds around the many different Microsoft virtualization offerings. Microsoft is working to make things a bit simpler by combining products into System Center Suite 2012, yet confusion still exists. Even with a bevy of combined products, you have to understand how best to utilize each, and also understand where one or another may not provide any value at all.
Our good buddy, Jeff Gilbert, just volleyed a link to a good source of information that talks, specifically about Med-V and App-V and goes over usage and benefits.
The long and short:
App-V lets you resolve conflicts that arise between different applications or different versions of the same application; MED-V, on the other hand, allows users to run older versions of Microsoft Windows concurrently with the local desktop of their computers, which can help with issues where legacy applications are unable to run natively on the most recent version of Windows installed on the user’s computer.
You can find that link to the full article here: Compare MED-V and App-V and Decide which is Right for You