In particular, in math it is clear which arguments are more dubious. DH4 arguments are often perfectly acceptable, as a simple and clear counterexample refutes a complicated argument that could easily have a subtle flaw.

The ability to make simple, irrefutable arguments is tremendously beneficial to sane arguing, for instance because it enables you to use the The Emperor Has No Clothes defense and avoid studying the details of your opponent's argument.

In this case, I'd even drop my initial thoughts about rudeness. If you can prove that somebody's gone down mathematical blind alley, it's downright polite to do so, since there's no ambiguity about the relevance of the steel man here.