I've always said that limits should be set to a maximum size to keep. Say no keeper small mouth over 15" or whatever. The keeping of the biggest fish hurts in a couple ways. We are removing both the eligible brood stock and the genetics required to grow that big.
Keeping only small fish removes the runt genes and utilizes fish that would normally be eaten by the lunkers that some one now has mounted on a wall, releasing small fish only increases the probability that the small fish genes will be passed on.
Once was a 4 1/4# brown in my little creek that over a three year time I caught and released over fifty times, one day I watched as a fellow caught her and and kept her on a stringer in hot sunshine where she was looking unfit to eat by the time he left the area a few hours later. The same group threw back several of those 11 1/2" fish that will never get any bigger than 11 1/2" due to genetics. All according to law, but she will never spawn again.
My thoughts are that trophy fishermen deserve to ruin our fisheries, they spend the big bucks to so, and that the agencies involved in setting size limits want smaller fish.