Just speaking from personal experience: A few years back, I happened to be standing near a group of college coaches watching a VERY good outfielder during a game. The kid was a STUD, but the back of his uniform jersey was untucked and the player never bothered trying to tuck it in for 2 innings. It could have been more than those 2 innnings, but I started paying attention to it when the coach pointed out to others.
Long story short, the coach left after those 2 innings and said "If he doesn't have any pride in wearing that uniform, he's damn sure not going to wear mine.". Three other coaches around him left too. There's a great article about this very subject on this very site...