It's not useless, although better self-defence systems exist.
Generally speaking, most combat sports aren't ideal for self-defence because they have restrictions on where and how to strike, as well as they tend to focus on not ending the fight immediately.
Self-defence ideally should focus much more on ending fights as quickly as possible.
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"Many of us like to ask ourselves, What would I do if I was alive during slavery? Or the Jim Crow South? Or apartheid? What would I do if my country was committing genocide?' The answer is, you're doing it. Right now." —Former U.S. Airman (Air Force) Aaron Bushnell