The lower front gondola gun was more for attacking targets on land or sea, but it was used for defense when possible. The nose gun was more of a deterrent to stave off head-on attacks, since it was an all-glass canopy (not really glass, you know what I mean), and was unarmored for the most part. It made the pilot extremely vulnerable. The problem was the extra firepower wasn't very helpful because it was slow-firing and didn't deter head-ons that much.
They were never meant for AH's tail-chase-full-throttle-drag-it-out-for-10-sectors nonsense.