Well, I think it's a shame, because the whole episode was damn good and it was ruined by the conclusion.
The idea of seeing human nature by an alien - Phlox - point of view was great and well done. The discussion between Archer and T'pol about how difficult it must have been for the vulcan with the humans was interesting and I really thought we were going to see a developement about when to interfere and when to stand by. But ! I thought it would be about the human point of view, about the relations between the two species (do we help the Menks to emancipate ?) or something about sharing technology and later on, see what problems it would cause to do so. I was excited to discover how and maybe why the sometimes appalling prime directive had to be applied.
But no, they went right where they shouldn't have gone. I could have "lived" with something that didn't live up to my expectations, but not an unethical end. And mostly out of characters for both Phlox and particularly Archer. The worst thing is, Archer gave Phlox the exact good arguments before changing his mind. We don't even know what made him change his mind. We don't know what T'pol had to say about this, not to mention Trip. Well, maybe it's best we didn't see them agree
I'm still puzzled wether the writers didn't really think this out or if they did it intentionally to provoke reactions about the PD ?
I think the writers didn't understand evolution or ethics very well.
And you're right. The idea that Archer would make this monumental decision without consulting anyone else, instead just being persuaded by Phlox's stupidity is kind of absurd.