On for a couple of reasons.
#1. Since I almost exclusively fly the CT, and I switch sides from Allied to Axis a lot, I am constantly in different planes with different ballistic properties, like jumping from a P-40 with .50's into a 109E with MG-FF's, HUGE difference there in ballistics. I never stay in one long enough to get that 6th sense of where the bullits will go if they are off.
#2. Probably tied to #1, my gunnery is attrocious. I can not take a shot over D250 and be confident of a hit, I have to have the tracers to help me quickly adjust.
#3. Having the tracers on is extremely useful in making a runner stop running and turn so you can gain ground on them. I do that all the time in planes like the 109E/F, both Zeke's, the 110c's, pretty much any plane with hugh ammo loads of small MG's. I run the main trigger for all guns, and another button (the one on the right side of the CH Fighterstick) for the MG's only so I can "spook" a runner.