Tracers are usefull in night combat. You can´t use your iron sights when it´s pitch black so you need to use the tracers to aim. And about giving away positions at day:
When we did combat shooting last year our group´s MG3 was also using tracers. They could be barely seen, basically only when you looked over the MG gunners shoulder. Only when we shot at night you could see them whizzing all over the place. And when you fire your gun at night tracers are one of your least problems because the enemy can see your muzzle flashes anyway.