The "double switch" involves bringing in someone like Marco Hernandez or Brock Holt, not David Ortiz or Edwin Encarnacion.
One of the biggest perks in baseball is that when you are down to your last inning you cannot just send your best hitter to get some hits and/or runs. You can give the ball to the best shooter in basketball, or draw a play to your best receiver in football, but not in baseball.
I say perk because that is one of the things I love about watching a baseball game, even if I get depressed when the Sox are down by 3 runs and in their 9th inning Travis, Vazquez, and Marrero are up.
My point is that it's not fair to exchange the pitcher with David Ortiz or Edwin Encarnacion because:
a. If it's their turn they get to hit, but an NL reliever gets to face Nolan Arenado, Giancarlo Stanton, Buster Posey, etc, in the same spot.
b. If it's the pitcher's spot coming up there will probably be a PH and the PH will not be a David Ortiz, but neither does the AL team have a David Ortiz ready to replace their worst hitter (like Marrero) unless for some reason that hitter took a break and he was called on (and this can happen in the NL too).
That was my main point when discussing relievers in the NL vs the AL. You specifically mentioned that they can have an easier 2nd inning and I can agree in the case of a starter. Facing a tough rally he can settle down by picthing around the #8 hitter and taking out the pitcher. But not so much a reliever coming into the 7th-8th-9th inning.