Is "parboiling" chicken stupid (or wasteful)?
Chicken breast is so lean and bland already that I'm not sure this method would really be that much worse than a different method especially when being shredded and mixed with strong flavors.

I don't think it's wasteful unless it is not eaten, but a far better way would be slow cooked and then shredded thigh meat. It is fattier, but in my opinion, a far more delicious part of the bird. It is stronger flavor wise, has an unctuous that chicken breast can't touch, and is cheaper.

Back on topic, if you really wanted to prove the value in either grilling or slow cooking the chicken breast. Do two batches next time: one half is yours and one half is theirs. You should definitely marinade yours if you want to win :)