The point is that the front turret does not have much to shoot at, and when it does those two .50s do not really make much difference, but of course it is at least something and their hitting power against a HO:ing fighter is better than that of those positioned shooting rearwards.
Maybe I'd go as far as stating that due to positioning the tail gunner has 40% of opportunities to fire, belly 25%, top 15% sides 8% both and the rest forward 4% (just a wild guess, though).
If you look at a typical closure angle to shoot down bombers in films of that era it is usually 6oc and slightly below.
-C+