I think every player is different. In some cases I'd agree that it might be best for the player to stay at FF and train with the 1st team. In others, it's best they go out on loan, whether it's 'successful' or not - Gordon's inability to hold down a regular spot in the Preston team this time last year may well have served to open his eyes to what he needed to do if he was going to improve, while Lewis Warrington is undoubtedly learning more in these months at Tranmere than he would have done staying at FF, playing at a level which he had already outgrown. He may well have got to train with the 1st team, sit on the bench, maybe even set foot on the pitch if we were winning easily with 1 minute to go, but I think he's better off where he is, and then come back to FF and try to impress the coaches during preseason.
Some players could well be damaged by something like Gordon experienced at Preston, others would learn from it. Everyone's different - it's up to the coaches to work out what's best for each player.