A lot depends on exit speed when analyzing launch angles. As you can see by the following chart, for example, launch angles of 25 degrees resulted in a .679 batting average if the batter was able to back it up with a 100 mph exit velocity, but dipped to .114 when only swinging hard enough to affect 90 mph. But, you can see here that you're correct in one respect, a 15 degree launch angle provides very good results across a range of exit velocities.