Nexus 7 with AOSP, what is missing from stock?
The Nexus 7, like most of the previous Nexus devices Google has released or been involved with manufacturing, runs the same AOSP source code that you propose to compile and install yourself, with only the addition of Google proprietary apps and hardware device driver binaries (though these are often available separately or in the source tree).

There are basically two reasons to do this:


You are a developer working on modifying the Android platform itself.
A fix for a bug that seriously impacts you is available in a slightly newer version of the source and it's so serious that you can't wait for the fix to be released normally.


If neither of these applies to you, then you probably shouldn't bother.