The truth about frozen thawed is convenience. Not everyone has access to live rodents of various ages so switching to frozen thawed offers advantages. It means you can order online or got to a pet store and buy quantities of frozen thawed rodents and store them in the freezer and therefore not need to either drive every time to a store to pick up a live rodent or find means to keep live prey for your reptile.
Sometimes however you have to do what is best for your snake. Some snakes do not take frozen thawed as they are either picky eaters or they have never been switched to F/T. You sometimes have to feed live in order to help them grow and gain weight. Sometimes you get lucky and can switch over on your own and in rare circumstances you can't switch.
As larger and larger prey becomes needed, the risk of injury to the snake increases. In a confined space, a snake can't escape a scared and defensive rodent. Time and time again we see the effects of inexperienced keepers letting live prey wreck havoc onto their snake.
If you have the choice with adult rodents, frozen thawed is always better for peace of mind.