There several advantages in using a minimal voting system:
1) The thread doesn't get cluttered by "+1", "I agree", "Good point" type of posts. Instead the posts themselves get highlighted by the number of positive/negative votes. That seems a pretty convenient way to identify what may look like the most interesting, or at least "agreed upon", messages. (and their opposites)
2) It's a convenient tool for admins / moderators to keep an eye on malicious posters. Yes, the negative votes can be abused, no doubt. But moderators don't depend on those for their decisions. They are simply an indication.
3) Not all posts are necessarily created for long discussions. When someone points out an interesting article or video people can simply add a like to signal that they have enjoyed the material.
4) I don't think that keeping score of the "thumbs up" / "thumbs down" for each user is very useful and I find it distracting too.
my 2c