You shouldn't. However, if an aggressive driver is behind you, safer to speed up a bit / move out of the way. Many drivers out there without valid registration / plates, no insurance (or extremely underinsured; state minimums are ridiculous low), and often impaired. Add to that the possibility of being armed.

More to the point, when getting rear-ended, insurance isn't likely going to cover much. Better to avoid conflict, but each to their own.