Well, actually they can cause problems if you use them excessively. Consider this from FM 23-9:
Do you also want to tell your enemy both where you are and that you are out of ammo?
Soldiers should avoid long-term use of 100-percent tracer rounds. This could cause deposits of incendiary material/chemical compounds that could cause damage to the barrel. Therefore, when tracer rounds are fired, they are mixed with ball ammunition in a ratio no greater than one-to-one with a preferred ratio of three or four ball rounds to one tracer round.
M196 tracers use strontium nitrate, strontium peroxide, barium peroxide, lead peroxide, magnesium powder, calcium resinate, and PVC for their tracer compound. The ignition primer is barium peroxide, magnesium, antimony trisulfide, and graphite. M196 tracer composition doesn't have much of a shelf life and it gets spotty after enough years. M856 tracers are much more impressive.
When fired, small fragments of tracer composition are likely to be dislodged and left behind in the barrel, sometimes lit, sometimes not. As tracer composition burns slowly (compared to powder) and hot (2000 degrees F) you really don't want it in the barrel in any great quantity for long.
Apparently the compounds can leave traces of ammonia, though not necessarily enough to be a problem.
Given this, it's probably a bad idea to have tracers in the last slots of your magazine also. "Tracer goo" could sit there awhile if you shoot your last mag and take your weapon home before cleaning.