Now I'm all for difficult content in the game. The best loot and achievements should only be for those who finish the harder content. But the story and lore should not accessible only by that way.
I think it was a brilliant to give players the option to see the story conclusion without going through the hassle of the group event. If a player wants all the rewards and achievements from the group event, they can go ahead and do it. But if you just want to see the story and don't give a rat's rear about the lootz, then this option allows those people to see it.
What's more, the design of the final Scarlet encounter is clearly more about the *presentation* of the story, not providing a gameplay challenge. And that is what it should be in order to make the story presentation more dramatic.
Remember the ending of Star Wars where Luke Skywalker crashed his X-wing over and over and had to do the Death Star trench run over and over until he succeeded? Of course you don't because that's a stupid way to tell a story. But that's how most difficult content in MMOs works out. And it always diminishes the presentation of the story by turning from something that should be dramatic into a grind.
ArenaNet has found an excellent balance IMO. They offer the challenging group content for those who want the rewards and the feel of 'earning' the story. But the actual finale of the story is a straightfoward instance that is really more of an interactive cutscene rather than a face-planting, grind fight. And they offer a method to access just the instance as a straightforward way for even the most casual player to share in the lore and story so every GW2 player can be part of the shared community experience.
Also, there is one other practical reason. I did the full experience helping defeat the bosses in LA to get to the ship and then beating all of the holograms with a group but when I went into the instance, my client crashed right in the middle of the closing cinematic. It would have been infuriating to have to wait again and go through all those encounters again to see the ending, Instead, I just needed to put on that backpack and I could re-run the closing encounter. I was very glad ArenaNet had a foresight to provide this option.