My experience has been that there's more to learn from better trees. A lot of what we're taught to do when developing trees, once you spend several hours with your hands in one, you can see how those decisions led to what's there now, which leads to knew things to learn compared with if we were still back in the developing stage. (My experience has also been that while there are several benches of world-class trees on hand, there are also trays and trays of shimpaku cuttings awaiting being twisted up and lots of trees in wooden boxes and on wooden boards. Project trees. Trees in development. A whole yamadori yard as well. Plenty of developmental techniques on display. I suspect most folks offering instruction would have similar projects at hand.)
There's also a pressure when you're working on their material not to screw up, ...though I'll admit those mistakes tend to be extreme one-offs as you're unlikely to make that same mistake again. I regularly splay my branching out too far on my own trees and so far haven't mastered how to increase the apparent age by having "more elbow" on my young trees, but I'll never again remove a branch off the top of a pad/branch without thinking about it after I screwed up one of his. (There was another one to cut back to close by, thankfully!)
That said, at 1-on-1 (or -2 or -3) workshops, with my own trees like the OP intended this discussion to be about, I've still managed to learn a ton. For me, there's something different about where my brain stores the things that I actually do with my own hands compared with what I read about, hear about, or watch someone else do. Having someone on hand to correct your form as you're working ensures you're not reinforcing bad behaviors. It's also given me an urgency to have trees WORTH having a pro work with me on.