Yeah, they had a pretty miserable time of it, didn't they? That's what happens when your navy is too outnumbered to do anything but "show that they know how to die gallantly", to paraphrase something one of the German admirals said at the beginning of the war.

The main usefullness of the bigger German ships after 1941 was that just by existing they tied up a lot of Allied ships and men and airplanes who were keeping an eye on them and trying to disable or sink them.

That's why, in my opinion, it was a good idea for the Germans to keep them in service. They served as a diversion for some of the Allied war effort, so it would cost the Allies more than it would the Germans, relatively speaking, to keep them around as a potential threat. The Allies, for instance, would want to keep at least 2 or 3 battleships tied up at any given time to watch one German battleship...plus a lot of airplanes and other ships.