Good question. Here's another one. Why cut it off? I leave the leaves in this condition as long as they're alive [note the green end]. I don't think the spotted part will hurt the plant.
I'd cut it off if I was taking the plant to a show (no need for other people to see my mistakes), but as long as it isn't spreading, I guess no need to remove it. Might make it worse by leaving an open cut for pathogens to get in.