It is a contentious issue Ginger I agree.
I think the opinion of barbless hooks causing damage is based on the evidence available given that no actual research has been done into it.
It appears that mouth damage is most prevalent, I would say without question, on commercial fisheries.
Not all fisheries obviously but when did you last catch a parrot mouthed fish on a river?
Most commercial fisheries, again not all, have a barbless hook only rule, or barbless above a certain size. Most I have visited anyhow.
If what I have stated here is true or is true in general then the evidence available points to barbless hooks or more accurately a combination of the hooks and repeated capture of the fish.