921,600 vs 2,073,600. That's the pixel count difference between 720 and 1080. Now imagine the weather is a bit crap, it's a bit grey, it's a bit rainy, and someone nudges you. You check your footage and you need to zoom in a bit to get all the detail on the number plate of the knobber that drove into you.
Wouldn't you rather have more than twice the amount of pixels to be able to do that? For general TV watching 720p is fine for pretty much everybody, but with something like this unless you give yourself the very best chance possible chance to capture a moment in time, it's verging on a waste of money.
The time when someone scrapes alongside your car and drives off, and the footage isn't clear enough in 720p, are you really going to be happy you saved a hundred quid?