I read a book about the US 8th AF/B-17 raids over Europe.
At least according to that source, the planners concluded that it took the fire from 10 machine guns (50 cal on a B-17) to effectively 'discourage' attacking German fighters. Hence the focus on tight formation flying, where multiple B-17s could bring guns to bear on an attacking fighter and (hopefully) discourage the attacker before he got close enough to be effective.
I think the typical load was one tracer ever 5 rounds - so having all tracers could easily fool an attacker into thinking 5 times that many rounds were inbound.