i have used the rear torpedo in convoys a couple of times. i will wait for the convoy such that a C2/C3 passes ahead of me, and a tanker behind. in most convoys there are tankers somewhere (near the center), so it's possible.
this way, i will fire 2 bow torps to the C2/C3 and (if i'm fast enough) the stern one to the tanker
. then dive and get out of there as soon as possible, as you are giving away your position like this. but it usually works. so, stern torpedos do have some use, i find.
see you!