I don't leave comments if I haven't read the whole Hub. In fact, I often don't leave comments if I have, because if I don't have anything to say that I think adds something to the discussion I won't add the old "good Hub" thing without anything else to say. I generally always (unless I forget, which I don't do too often) vote a good Hub up.
If I think I have something that might be worth saying on the person's Hub, I'll say it. Sometimes I do longer comments, and not everyone appreciates those; but I figure they can just not-approve the comment if they don't want it on there. Personally, I like it when someone takes the time to write a long, thought-out, comment (and contribution to the Hub).
I think if I ran into a Hub that I'd see as great reading (but didn't feel like reading at the time) I'd probably vote it up and leave. If people go around leaving meaningless (or close to meaningless) comments on too many Hubs, they're likely to see their score drop; because HubPages discourages that.
In fairness to some people who make comments that make it seem they didn't read the Hub, just the other day I went back to a Hub that I had read and commented on. It turned out I completely misinterpreted where the writer was "coming from". I went back to see if I'd missed something when I read, and I couldn't see that I'd missed anything. So, sometimes it can look as if a person didn't really read when, in fact, s/he did. :/ BUT, in that case, I think someone would see from the substance of my comment that I had missed "where the writer was coming from", rather than believe I didn't read the Hub at all.