My game version (SH3 + GWX + SH3 Commander) does not let me return to port by exiting patrol if played at 100 % difficulty. I have to be 25 kilometers from the port to end patrol.
Sailing back to port is dangerous from 1943 onwards, because there is a lot of air activity. Running out of torpedoes doesn't mean there won't be any action. In fact I think getting back might be the most difficult part of the mission. I might be running low on fuel, AA ammo, BOLD capsules or even oxygen capsules (I have just begun using Hsie's realism mod too, so I have to keep that in mind too). I like evading destroyer's, calculating courses, evading airplanes, so the return trip is fun too - though I admit I a nice torpedo hit on a target is the best part of the game
On the subject of renown, however, I think SH3 has never given renown for returning to base, as it was supposed to be - the threads here say that the routine of giving renown points for returning were bugged.