I kinda agree with you, except that if someone drives insanely enough to endanger my life, having "the cojones to do anything in person" would entail driving my car into theirs. As often as I've considered doing just that, I've never actually given in to the temptation.The morning commute for Dallas is not for the faint of heart and anyone so miffed that they would spend the time and money looking up my personal information just to send a letter is just some passive aggressive schmuck who doesn't have the cojones to do anything in person.
Overall, the person who sent you the letter was doing about the best they could to "have the cojones to do anything", considering the circumstances.
Now, I do know of a PI who would call his office, run a plate, cross-reference with phone data, and get the cell number of any driver who pissed him off. Then he'd call that person, tell them they were driving like a jerk, and inform them that he was on their bumper. Then he'd wave at them. They often pulled off on the shoulder in fear.
That's going too far.
So the guy who sent you the letter seems to have found a reasonable middle ground between "doing nothing" and "stalking and/or assault".