I agree that this is the right way to think about it: I upvote comments I think others should read (for benefit, enjoyment, or a well-reasoned counter-argument to my view). Sometimes, however, due to the discussion at hand I simply use the vote to indicate "me, too" -- especially when it's a discussion that seems to be coming to (or trying to) come to a consensus. I certainly don't want to submit an "I agree" comment; I just up-vote. It often seems like I'm up-voting agreement more than the well-reasonedness or interest of the comment, often due to the nature of the topic.

For example, on a "rate my web site" thread where somebody makes a suggestion, and I simply agree, I'll up-vote without adding to the thread.