They could arrest you and accuse you of assaulting them. Your argument is a straw man argument.
Well, I see unlawful stops like this make the news all the time. Details are sketchy since its still being argued in court and no one is talking but a guy is now dead because he was unlawfully arrested for having a knife. Again, details are sketchy, but as I recall the guy was overweight, diabetic, had mental issues, and was not properly restrained when placed in the police van. He ended up injuring himself and his medical issues compounded the problems. That is one extreme example of many that don't result in dead citizens.
I'm saying the duty to inform empowers police to fine a person for "contempt of cop". It happens. We could do less with laws that make it easy for less than stellar cops to make our lives difficult.