The only thing that leads me to the conclusion there is some degree of benefit is that you see a lot of semis on the highway with aerodynamic fixtures on them. Not necessarily Air Tabs, but it is becoming very common to see this stuff:
I find it difficult to believe that trucking companies would go to all of that expense if there was no benefit.
While this is a somewhat different approach than the Air Tabs, I think the drag issue is valid, whether the Air Tab design is the best approach to reducing drag or not.