So let me get this straight, a player on your team has been able to show you something at a tryout that they didn't show you in the 50-100 games that they already played for the team? The ENTIRE season was a tryout for them. Can you honestly tell me that you ever changed your mind about a player already on your team based on a year end tryout?
I think the idea of inviting a player to a tryout is more symbolic of 'you are not entitled to a spot on the team just because you played here past year.' If a better player shows up they will get the spot, if not you will be given the spot. Its a lot easier to say every player tryouts every year rather than having them hold to see who shows up at the tryout.