While I agree that ground can affect a detectors performance greatly compared to the air test, I also believe you can tell a lot from the air test.
First off NO DETECTOR I've every owned was able to consistently go deeper in the ground than in the air test. NONE, NADA ZILCH. Yes under the right conditions I have gotten deeper in the ground than in the air test, but it was not much deeper, and very seldom. And that also includes the Minelab Sovereign which many claim is the exception.
Second, if your detector has target ID its just plain stupid to not do air tests to learn the response of various targets. Sure on targets deeper than 5" that VDI won't be near as accurate, but on the 5" and less it will be extremely accurate. So air tests also teach you what to watch for on your VDI and listen for. It also allows you you make adjustments and immediately see/hear how they affect a detectors response. Again, I realize it works best on 5" or less targets when in the ground, but it still works just fine. What percent of your digs are more than 5" deep? There ya go!