Reward already received = you are wasting your time.
It would be nice if there actually still was some kind of bonus for doing content you no longer need. It can feel highly unrewarding when you are just helping someone else to do content you shouldn't be doing for the next 24/7 days till reset, as it is no longer effective.
Like doing roulettes etc. When you already have it, there is no difference between picking a good dungeon and using it while risking a bad dungeon. The reward is the same.
Alexander raids are even worse. Farming your loot is hard enough with this loot system, but it gets even worse when you have to farm it even more to help your friends to loot. It can feel really annoying at times seeing them loosing rolls again and again, while you are there just to help them. To be honest, I would even like the option to pass all the loot and give a little roll bonus to one of the other players.
So it would be really nice if the game still rewarded something for doing content where you can't be rewarded anymore at this moment.