Dad did feel the front turret on his Lanc was of no value for night ops- in 19 ops he never fired the guns in anger, nor even saw anything to fire at! As pointed out, his duties as Bomb Aimer only lasted for about 15 minutes preceding the actual dropping of bombs. During the trip, he assisted the Nav in observations and navigation, or was a general dogsbody when something was needed throughout the aircraft ie oxygen problems.
That being said, I did read once about how a front gunner did actually shoot down a night fighter. The enemy aircraft was unaware of the Lanc, and cruised past just a hundred yards or so parallel to the Lanc. The mid upper became aware of it as it was abeam them, and notified the front gunner. When the fighter got in a position slightly ahead of the Lanc, the front gunner shot it down! There may have been other instances, but I am sure they were certainly few and far between.