It would be pretty stupid to spray pepper spray in someone's window while they are driving...I don't think that was the intent of the suggestion...but to have some kind of defense if someone got out of the vehicle to confront or attempt assault on your person. If you picked up a stick and hit 'em in the head, you could also be liable. Shooting someone can also make you liable. Hell, any kind of defensive action is subject to liability, but it would sure be better than getting the chit beat out of you or being dead.
One could debate the merits of ANY defensive action and the potential repercussions. Bear spray is a pretty good alternative to deadly force, IMO. In a bad situation, lots of things can happen, but doing something is better than just laying down. Of course, fleeing is the best alternative...