Zamora makes it clear from the blog the benefits to this approach:
To better serve your needs, the Windows Phone Dev Center now offers the ability to associate multiple XAPs with the same app GUID. This means that you can create, submit, and maintain different versions of your app for Windows Phone 7 and Windows Phone 8.
Developers can still work on a Windows Phone 7 code base and ensure that it is compatible with Windows Phone 8, or simply upgrade the Windows Phone 7 code to run solely on WP8, or to have two XAP's for 7 and 8. The worry here is that new developers will focus solely on Windows Phone 8 and that older devices will not receive the latest applications. In the short and medium term, the installed base will be built on WP7 and you;d be a fool to ignore it (unless there is a specific code function only available in WP8).
It's at this point old-time Psion and Symbian developers will point out the SIS installation files on the older platform always allowed code bundles for multiple machines to be in the one file, but let's not upset the apple cart too much.
More details and a walk-through of the process can be found online at the Windows Phone Developer Blog.