Just in the scope of the game, you have a limited supply of torpedoes on board. So why spend them on DDs which will give you around 1000 tons and many chances of missing when same torpedoes can bring down a 10.000 tons freighter ?
I give it a try only when I have acoustic homing torps in the rear tube and can send a long running shot just before a crash dive. Works quite often...
PS : if you are caught shallow by DDs you can bite yourself in the arse because you should never have been there...