During the TechEd 2012 event, a session on the new features of Windows Phone 8 (WP8) clarified the use of microSD cards for apps. When announced last week, the slide for microSD support showed that the new feature supported storing media (photos, music and videos) and "installing apps". This refers to installing apps from the card to the phone, and not to the card directly.
Primarily a feature targeted at enterprises, this feature allows companies to bypass the Marketplace (the Windows Phone app store) when installing apps specifically for the organization's use. This makes it easier for IT departments to create and deploy programs.
Microsoft is selling to the IT department not to the end user.