I can't speak for it being immoral, but basically, using rude and offensive language tends to undermine rational discourse. If your purpose is get at the truth, then it is prudent to avoid such language.

However, a lot of writing (academic, journalistic, what have you) has "getting at the truth" as only one of its goals among many, including "expressing my feelings," "attacking someone else," "playing with poetic language," "sounding smart," etc.