^See, I don't think it preached anything. Preaching implies asserting a single, unambiguous, undeniable right answer. "Dear Doctor," to me, plays as just the opposite -- it shows two moral people from different cultures and value systems trying to come to terms with a deeply ambiguous question and sincerely disagreeing with one another on the right path to take. Archer's ultimate choice is made reluctantly, and it's left deliberately ambiguous whether it was the right choice.
Personally, I would've preferred it if the show had dared to tell a story where Archer and his crew did recklessly intervene and things went horribly wrong as a result. I wanted to see these novice explorers making mistakes that had real consequences. So I was always a little disappointed to see Archer trying so hard to obey the dictates of what would eventually be the Prime Directive, since that felt like a missed opportunity to explore the serious screw-ups that would've realistically been part of humanity's learning curve. (Although I think "The Communicator" did a fairly good job of showing how badly Prime-Directive thinking itself can screw things up, though that wasn't the intended moral.) So in that regard, I wasn't entirely happy with the outcome of "Dear Doctor." But the outcome is not the only thing in the episode. Like I said, I think it still has plenty of merits to its writing, characterization, and production, even though it has its flaws as well.