Suppose that I don't have the ability to upvote my comment, but I have the ability to upvote someone else's. So even though I can't say that my comment is better-than-average, I can say someone else's is. Thus, if I think my comment is the best, I shouldn't vote anyone else's comments higher.

Okay, I was reaching a bit with that one. It could, after all, be argued that whatever concern I have in that situation, is identical to the situation where everyone can upvote their own comments and always does so.

However, in the situation where everyone doesn't do so (which would perhaps be more likely if karma was not tied to the auto-upvote), I have the option of re-evaluating my comment in light of other people's, and removing the upvote (or even downvoting!) if my opinion of my own comment changes. In such a situation, upvoting someone else's comment just means, "This comment is as good as mine".