You don't engage in Steelmanning to be nice. It is for your benefit.

Demonstrating that you fully understand an argument better than your opponent, and being able to charitably represent them, just makes it all the more devastating when you knock them down.

If you really disagree with an argument vehemently, then you should steelman it. Strawmanning makes you look dishonest and fragile, only capable of attacking a weakened form of your opposition.

If someone who has really abhorrent opinions tries making their argument, then you are letting them win if you strawman them.