I think we're seeing this exactly the same, just articulating it slightly differently. My point here is that the one merino and one set of long johns you are going to be wearing can be factored in when deciding how warm a bag you need on that trip.
For a person that is relatively more willing to "hop into the bag" when cold, the "worn" warmth layers can indeed be reduced. OTOH, for a person who prefers, or for a situation that demands relatively more warm clothing, then the weight hit from that can at least be somewhat offset by a lighter bag.

That, of course, assumes sufficient granularity of choices between various sleeping bag options at home --- something that more normal (sane) people likely have less of! :-)