Ability to replay personal story
There are a lot of stuff to do after reaching level 80. You could just do those. Or create other characters so you can have all the crafting jobs. Why would you want to replay the story missions on a character when you can create another one and explore a different path? You can always check the wiki for possible choices if you aren’t afraid of spoilers.
I’d do it just to go back and listen to Tybalt.
Even better than replaying your story, is making another character and seeing a different story and seeing out they interwine…because they do.
A lot of the stuff at the end of the story is better understood by playing multiple races.
Although I doubt there are that many people who would want to do their story quests again, there is no downside to allowing it. Would be nice if you could find a NPC in your home instance who lets you replay any of your completed personal missions, similar to the Scrying Pool in Eye of the North. Similar to the Zaishen Missions in GW1 they could have a daily rotation of personal story missions with decent rewards such as laurels for completing them. This would give players aanother incentive to visit the home instance and also make it easier for people looking for groups to do Arah story with.
While I can make a new character and explore a new path, as I’m doing right now with my necromancer, some quests I’d enjoy to go back and do them again. I’ve enjoyed both the vigil quests and the whispers quests so far, but I wouldn’t want to make a new character just to do those again.