This is very true. It’s the classic “argumentum ad personam”, as described by Schopenhauer as the last trick, if all else fails.
“A last trick is to become personal, insulting, rude, as soon as you perceive that your opponent has the upper hand, and that you are going to come off worst. It consists in passing from the subject of dispute, as from a lost game, to the disputant himself, and in some way attacking his person. It may be called the argumentum ad personam, to distinguish it from the argumentum ad hominem, which passes from the objective discussion of the subject pure and simple to the statements or admissions which your opponent has made in regard to it. But in becoming personal you leave the subject altogether, and turn your attack to his person, by remarks of an offensive and spiteful character. It is an appeal from the virtues of the intellect to the virtues of the body, or to mere animalism. This is a very popular trick, because every one is able to carry it into effect; and so it is of frequent application.”
http://coolhaus.de/art-of-controversy/erist38.htm
It only shows how weak our opponent actually is.