Why YSK? Comments you reply positively to, should definitely get your upvote. Comments you disagree with should be at your discretion. Trolls deserve downvotes (seriously, they live for those). Disagreeing with someone in the midst of a good discussion doesn’t necessarily warrant one, and might deserve an upvote. Even if you don’t reply, but you agree with the comment, give it your vote.
Also, this has nothing to do with propping up folks’ egos. Comments with more upvotes will likely be seen first the longer the post is up. Alternatively, downvoted posts are less likely to be seen unless users are looking for them.
Of course, this can lead to folks accusing communities of having leftist/right-wing bias, but I think overall it improves the usage of the site. Personally, the thing I liked most about Reddit was the conversations in the comments. Usually the ones with the most upvotes were more worth the read & engagement.
In fairness I didn’t make that point, someone else did.
I still agree with the OP though.
It’s completely irrelevant who made the point. If you can “agree” or “disagree” with the content, by principle it does not belong in YSK.
Take a look at some of the posts in this community. Does it seem like you could agree or disagree with most of them? No, because the typical YSK post is just a plain piece of information, which is either true or false (hopefully true).