Exactly, there are guidelines and things which the police have to follow. Even if the person they've pulled is acting like a total fool, the officer can't just fly off the handle without due reason. If you're acting like a dick fine, you're gonna get whats coming to you. If the officer has just had a bad day and wants to rip the person's throat out, then thats a different matter.
If they you are in the wrong, sure, recording things won't help. But if they are acting in an unreasonable manner too many times if a recording of the situation was done it would have helped later. I've know people who have been physically abused by police for even just a verbal comeback - now if you think that is how a cop should be acting then fine.