My own personal experience with this hasn't been that great. I've lost a few flies to small trout in small ponds, and when I came back to the pond the following day, there was my fish floating belly up. I knew they were my fish because they had my flies in their mouths. I fish barbless, too. This has happened to me often enough that I fish with fairly strong leaders. I may miss a few more fish as a result, but I really have no desire to have them die because my fly is in their mouth. I haven't seen anything like this in rivers, probably because the fish float away.
On the other hand, I once lost 4 flies to one steelhead and got really mad at my knot tying. I triple-tied the next knot and hung on to him. I plucked all five flies out of his mouth and let him go. I was surprised that he kept coming back for more - sadist! My brother-in-law had a very similar experience with a steelhead.
Perhaps the size of fish matters in this? I would like to see a study done on this.
Dryflyguy