There was a time in the early 90's where offensive linemen Mike McGlinn and Winston Sandri were kicking serious butt in practice, but when it came to gametime playing, both were basically spot starters when injuries popped up among the regular starters. They simply weren't as good as the other starting linemen. Both Lou Holtz and Joe Moore knew his, which is why the offensive line starting lineups during those times were remarkably stable (barring injuries).
Also, in 1991, all of the linemen would tell you that Reggie Brooks was absolutely destroying everyone in practice, and that he was easily the best back on the roster (despite the fact he had been a cornerback for his first two years), but since his older brother (Tony Brooks) and Rodney Culver were already established, had to wait his turn. He did get one start, though, when both Culver and the older Brooks were banged up one week, and turned in a very nice 100+ yard performance on just a few carries.
That being said...
Lou Holtz would give sometimes give practice standouts a shot, with the understanding that there were guys in front of them who were better, and that they could be yanked at any time. He wouldn't die on a hill with them, though.
He did this to Todd Norman in 1993, where McGlinn was given the start against Pitt (and did decently), to motivate Norman to have better practices. He did put Norman back in for the second half, and it certainly worked.
The opposite of this was Bob Davie, where he'd stick with the practice standouts, even when they basically lost the game for us. The worst case of this was when he stuck with Jim Sanson as the placekicker, simply because he was the best in practice. Sanson's shanks made the difference in several of the games during the Davie tenure... He had better kickers on the bench, but refused to give them a shot. During the pressers, he'd say that Jim Sanson was easily the best in practice and that it wasn't even close.