"I agree" is useful in cases where people think that their agreement would provide me with a non-negligible update, because they know that I respect their rationality or because they consider themselves experts in the domain in question. For example, an "I agree" from thomblake would provide me with non-negligible evidence if written in response to some speculations about academic philosophy. Of course, most people don't signal agreement for this reason, so it's not really what you were talking about.