Actually, I recall reading that some units DID ask for them to be removed from their belts, so in effect they were turned off. I'm also remembering that the unit had an increase in kills. (Take that with some salt -- there are days I dont remember where I left my keys!)
Now, about in game tracers -- I flew for several years with them off, because I couldnt keep my eyes from following the bright streaks instead of watching the relationship between the pipper and the target.
Unfortunately, I hit a hard "tracers off" plateau and about stopped improving my gunnery at all. I could hit the shots I "knew", but couldnt learn what I was doing wrong on the rest of them...so I have them back on. This time, though, I'm mentally disciplined enough that my focus remains on actual targeting data (pipper position, degree of lead, target aspect/angle of flight, target size/distance, target speed -- and yeah, you HAVE to take al lthat into consideration to know where to fire). The tracers let me see immediately what I did wrong with a shot I missed, so I have been "learning" new shots. I agree with Murdr, though, that I'd prefer them off once I've improved my gunnery enough to consolidate the new shots into the "automatic" zone.
So for me, the answer to the tracer question is Yes, No, and Sometimes. Don't let yourself use tracers as a substitute for aiming -- but otherwise, use whatever setting helps your game at the moment.