"Some kids are gamers". "Give him a chance" Ive heard that for 25 years as a coach lol. Ive had players who were not good practice players but excelled on games day but its not often enough to rely on that. How do you know who the gamers are? Should we go into compettive game and play random players to see who the gamers are? No, as a coach you have to rely on what you consistently see first. Players need to show they can be trusted. A lot of the times coaches see the athletic potential of player but also see other things that raises red flags. The type of things that can harm the overall production of a unit. These are the things fans can't see. These guys make a living coaching at that level. Playing randoms to see if they are gamers is not a smart strategy at all. It's cool to do that on a Madden football game but not in real life.
Yes, playing live games build confidence and experience. I also depends on the situations in the game. Garbage time doesnt develop much. You might see a kid flash here and there and that may allows you to give him increased reps in practice but you are not going to win many games trying to experiment in competitive games. It becomes a bigger show. Now you just throwing different players in to see how they do. If you think a kid flashing during garbage time is a strong indication of how his career is going to go than I dont know what to tell you.