Oh my lord people...
Tracers off/on option only affects YOUR tracers, it does not make other players tracers invisible on your end. YOUR tracers are invisible on THEIR end.
In real life, most units stopped using tracers at various points in the war because-
A) Tracers give you away if your first shot is off target. As it almost always is.
B) Tracers give you away to everyone else in the enemy formation even if you do get the guy you are shooting at. Unlike in AH, most real pilots didn't charge around alone, and actually looked around once in a while.
C) Tracers have different ballistics than standard rounds, which means if you are using them to sight the 4 rounds in between may be overshooting or falling short.
D) Its just plain easier to load all those belts if every round is an API than if you have to count them and insert a tracer every fifth round.