Do I need the 1.3 patch for Honest Hearts? I'd prefer not to use it, because it works fine now and I'd rather not take the chance of changing that.
But do I have a choice, I think it automaticly downloads when I'll grab Honest Hearts from Steam?

Yes, you need the patch 1.3. Otherwise, you can't play Honest Hearts.

patches generally aren't optional when i comes to new content.