Since there hasn't been much study on this how bout this for a theory... Using a heavy rod and ripping a fish in might damage the mouth or be more likely to rip the jaw or cause bleeding. A lighter rod might actually flex when a fish does a head shake preventing damage to the fish. I am just throwing in something to think about, I don't prefer ultra light or ultra heavy rods when I fish. I fish with the rods I like and try to take care of the fish as best as I can to ensure survival. At the same time if I ever doubted a fish would survive and it was legal size I would take it home and fry it up in some butter. I think catch and release can be taken a little too far at times and I RARELY if ever have taken a fish home with me for any reason. I think we often forget that fly fishing and fishing in general has its origins and bringing in a meal. Nice to get back to your roots now and again.