Rosters expand to 40 in less than a week. Every team takes advantage of this rule, as it's perfect for teams in the stretch run to gather reinforcements. The New York Yankees are in this category.
There are other categories as well. Middling teams can put youngsters in relatively high-intensity situations to see how they fare. Bottom-dwellers can give their rookies playing time in order to prepare them for the future.
Not all 15 extra roster spots are used, though. Some teams, like the Yankees, for instance, might not call up more than five players. They'll stick to the essentials—a third catcher, an outfielder and some bullpen help—but they likely won't call up kids for the sake of calling them up.
There are five notable prospects who more than likely won't receive a call on September 1. These guys are some of the team's top prospects and will certainly have an impact on the team in the future. Just not in 2014.
Note: To be considered for this list, the player had to be in at least Double-A Trenton, seeing as jumps from A-ball to the majors almost never occur.