It's all personality based. For some, just witnessing the death of a kitten is a traumatizing event. Or even the end of a miserable Kobold thief. They don't actually have to kill anything.
Yes, I have witnessed workers drinking extremely frequently after acquiring the trait "doesn't really care about anything anymore". I am of the school of thought that the effects of the trait are also related to personality, ie a person who was particularly upset by violence would become a drunk.
Some people have postulated that military experience and/or status (recruits, champs, civilians) might account for the difference.
My personal experience was with two dwarves, one an architect/mechanic and the other a mason. I needed a pump built shortly after a siege, and my architect literally left the job 3 times to get a drink without bringing 2 components to the site. This was after witnessing a siege, where before he had built several more distant pumps quickly and without interruption. He had 3 legendary skills, so he was certainly fast enough. He seemed to become more functional later. My top mason at the same fort displayed similar behavior, and has yet to stop drinking frequently each season.
The reason this is not considered hard fact, and is the object of much speculation on the forums, is precisely because it is something that is hard to pin down and confirm.