I see your point, but I don't believe we have the same definition of "information-free". Unless I'm mistaken, upvotes are anonymous. A comment of "thank you" or appreciation is essentially a publicly identifiable upvote. The information being conveyed is to the commenter and that information is the appreciation of the comment from an identifiable individual.

In just about every situation you could say that greetings, salutations, etc. convey no obvious information. But I'm not sure how far we'd get without them. I personally believe that things like this are what maintain and bond a community. But that is simply my opinion. I'd take a thoughtful comment reply, even if in disagreement, than an upvote any day.