It doesn't actually mean it is useful or even accurate in any way - it just means you are doing something. You could probably put the energy into managing the environment or better communication or coming to terms with "Other people are complicated".
I think people just find the entire idea of labeling humans in this way offensive, and latch onto some of the claims that it's "science" as an avenue of attack.
Edit: And I think it's slightly better than just categorizing people based on their birthday like you do in Astrology, as you are using at least responses to a personality test in most instances.
I'd say that astrology is better than DISC in that it's obviously bunk, the problem with the colours is that while some people claim to enjoy it as a pastime or conversation starter, some seem to take it further and base actual decisions on it. People reading horoscopes in a magazine wouldn't typically actually follow the advice therein.
How do you know? See also, by the way, Japanese people’s beliefs about blood types:
https://en.wikipedia.org/wiki/Blood_type_personality_theory#...