How should I decide whether I need a rear cam?

Having a front and rear facing cam recording at all times can easily win you insurance claims if you were in the right. Just make sure they don't record audio, and don't take telemetry off your car, I.e. speed etc.

Friend of mine had a rear-end claim, and where he lives if you rear-end someone you're automatically in the wrong. Not enough room blah blah blah. Thing is they were at a red light and someone rear-ended him into the back of the person ahead of him and took off. Insurance company was just going to make him at fault automatically until he produced video of the other car smashing into him and taking off.