Even in tight races, September call-ups are usually not going to make or break the outcome for your squad. Any player who could have a significant impact is generally already in the big leagues.
For major league teams, both contenders and noncontenders, Sept. 1 is generally about roster depth. That usually means third catchers, utility infielders, more bullpen arms, extra bench bats and veteran minor leaguers getting the reward of a cup of coffee in the bigs.
While some are prospects getting their first taste of the majors, other players who join the expanded roster have already seen time in the big leagues this season and are returning. (Domonic Brown, Pedro Alvarez, Edinson Volquez, Mat Gamel, Chris Carter, Jerry Sands and Travis Snider, among others, fit this description.) Sometimes players have to wait even longer than the first of September if their teams are in the minor league playoffs.
I've said this before, but the most important factor in figuring out the impact of a September call-up is potential playing time. Just because a player is called up, it doesn't mean he's going to get enough playing time to be a factor in fantasy. In mixed leagues, a player getting only part-time at-bats can put an owner at a disadvantage. AL- or NL-only owners can more easily accept a part-time player if he has a chance to be productive, but mixed-league owners need their hitter to be in the lineup regularly.