That can't possibly be the reason though. I mean, just look at the rules that come with Seaside. That's not even a confusing point. The drawing of 3 cards very clearly happens this turn, not next turn. That's not controversial, is it? This is why an attack like Militia has teeth.
Of course the drawing happens this turn. Certainly no one questions that! The issue is that the drawing happens after Outpost would have been otherwise discarded. At the time that you need to decide if you should discard Outpost, it's not done doing everything it needs to do.
I do agree with you that the wording of the Duration rules seems to imply that it shouldn't matter if Outpost is completely done or not; only that Outpost cares if it's going to be done this turn or not.... I guess I'd say that the rules disagree with what Donald said in that post. Although there is, and never will be, a situation where the question comes into play.
Actually, there is another issue here... Outpost doesn't do anything at all next turn. Causing you to take an extra turn is something that happens at the end of your current turn, not something that happens next turn. So, in fact, Outpost, by the reading of the rules as you read them, really shouldn't stay out at all.
Just remembered this thread:
http://forum.dominionstrategy.com/index.php?topic=1585.0Donald goes into more detail here about why Outpost stays out. Though again, I see how you can argue that what he says goes against the Seaside rules...