We paid for our air spade with one job Squish...a school board job where I radial trenched 24 trees. GREAT tool...but not so hot if the sod ain't moved. I'd say pick it up. Since we bought it, however, the jobs we sell for it are few and far between...but I'm sure you'd see the opportunities much better than my employers have. The nice thing is that if you can justify the purchase of the large compressor, you could have a fella driving around winterizing irrigation systems in the fall and cleaning them out in the spring, golden nugget...which keeps a guy working.
It opens up possibilities especially if you want to get into bare root transplants. With the big nurseries you have nearby (Grindrod, Enderby...right?), it might be a nifty way to open up a niche market for bare rooted trees.
My advice, do it. It's a great tool for which a mind like yours would find many uses.