Most of the time people who are "loser parents" don't care what anyone else thinks of them and would get angry at anyone who presumed to tell them what they shouldn't or should do. Having said that, though, the way I see it is this: Such parents don't think twice about embarrassing and demeaning their child in public; so a dose of their own medicine (while most likely useless, other than to make them aware they should be rotten to their child behind closed doors) isn't such a bad thing.
Maybe it's not such a bad thing for a child in that situation to have someone step in and send the message, "You know what? This isn't what parents are supposed to be doing. That person who is treating you this way is wrong, and someone in this world sees it and will say something about it." It won't help the child in his immediate situation a lot, but at least there's a shred of a chance that he'll get the message that he's not the one who has something fundamentally wrong with him.
Besides, who knows - Maybe the parent is someone who actually doesn't realize how "bad" his own behavior is. Maybe it would be something that would make him at least stop and think, or maybe it would be an incident he'd vent to someone else about (and that "someone else" might have a chance to point out that the stranger wasn't all that wrong).
Last week when "this moron" killed her little six-year-old boy at a hotel, people said they'd heard her screaming at him that afternoon. To me, saying something, doing something, (especially if someone's stupid enough not to know what shaking a child can do) is always better than doing nothing.
I'm the biggest one in the world for minding one's business when it comes to other people's kids, but parents who are behaving abusively toward their kids give up their right to have other people mind their business.