I understand that "honesty" as a general statement might seem like a useless rule (although it really depends what you mean by rule. If you mean principles you live by, then it's a great one) but more specific things like "I would like to know you're interested in someone before you act on it" or "Please let me know when you're not going to be home for the night" can make a lot of sense, because if you don't state these things, the other person might never know you wanted it, and they won't tell you these things, not out of dishonesty, but because they don't think about it.
Even if something seems obvious, I find it's a good idea to talk about it. Some people want to be told some things specifically, some people want NOT to be told some things specifically. If you don't know what these things are, you might hurt each other accidentally.