Try getting tailgated by a semi on a wet day on I-5 sometime. On a dry day, I could have lost him easily. On a day with wet roads, I really didn't want to sustain outrun speed for long.
Never knew what set the guy off, or if nothing set him off and he just thought it was funny to get right up on bikes. Either way, when a driver appears to go out of his way to tailgate a bike, it's very threatening to the biker.
And no, I would not want to try to shoot from a bike against a paper target (balance could get interesting, as I have never practiced that), let alone at a multi-ton vehicle that might decide to become a battering ram.
But evasion isn't always a ready option. Depends on location and conditions.