Repeating One-Time-Only Quests

I see a lot of frustration caused by a few quests that some players are unable to complete solo, and others cannot re-enter these areas to help once they themselves have completed the quest. Examples include Pierce the Shield Wall and Repository of the Three. As these are critical quests, players cannot simply skip these because they cannot find a group.

If these quests were allowed to be repeated, players would be better able to assist each other, and it would relieve a serious playability block for those who are having trouble getting past these.