So here is my guess at why no checkpoints. Sure Bungie does not like checkpoint abuse, but as we all know, that's a weak argument since as you read replies in this post, clearly there are easy ways around this. (Like the passage card, checkpoint keys, whatever). The REAL reason it isn't implemented is exactly the same reason there is no Optional matchmaking for high level content- BUNGIE DOES NOT WANT THE MASSES TO FINISH ENDGAME CONTENT TOO QUICKLY. Period. So they create these artificial walls/barriers to do so. It's the proverbial "dangling carrot" in front of our noses. They know matchmaking or PoE checkpoints would make finishing the missions easier for most. They know it's a huge pain to get a raid fireteam... (Without in game matchmaking). They know it's a huge task to have to defeat Skolas in one sitting... (Without checkpoints). Give us checkpoints and we will all eventually have the Skolas checkpoint and will be able to take him down much easier at our own pace. Just like we can do with Crota or Atheon having those checkpoints. It's Bungies way to restrict access to endgame.