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.
Fair point. Have an upvote! ;-)
It’s certainly not a perfect system, and as has been pointed out by other comments I may be completely wrong about what up-/down-votes even do. Then again, if they don’t do anything, why have them at all?
I just want to point out, that I use the voting system just as you’ve described in your OP. Upvote if I strongly agree or find funny etc, I hardly downvote, I save that for the more malicious stuff, and I comment if I feel the need to add or discuss. So I am in agreement with you, but I realize there is no right way to do it, it just makes sense to me (: take my upvote