I'm not sure if the police would act on a video you took to them. I could be wrong about that though.
I see no harm in having one and they sure can come in handy in protecting me.
Oh, by the way, I have a gun for protection.
It's better to have one and not need it than it is to need one and not have it.
I agree that in the rare instance you need one, it would be nice to have the video to back you up.
The logic behind "better to have one and not need it..." is false and always has been but people use it anyway. Using that logic, we would not be able to leave our house since we would need to carry so much stuff with us because we just might need it even though the chance of needing it is very remote. For instance, do you carry an umbrella with you on a sunny day? No, of course not, because the odds are low that you will need one that day.
I apologize if I'm coming off as saying you shouldn't have a dash cam, I don't mean to be coming off that way, I am just stating reasons why I wouldn't use one. I might regret my decision one day.