The bags they operate with don't have stoppers so you waste a lot of vapor because you have to cover the holes with your thumb and you inevitably lose some vapor whenever you hand it off.
I've been using the herbalAire for a couple of years and I'll have to strongly disagree with your 'wasting a lot of vapor' statement. Sure it's nice to have a valve, but covering the stem with a finger (yes, any finger will do) is not a big deal. And once you've taken a hit and equalized the pressure in the bag, vapor loss is minor even if you don't cover the stem. Without a difference in air pressure, the vapor pretty much sits in the bag. The Extreme would be the same. It's a minor drawback and quite possibly a price difference of $240-300 would be more important to some.