As for the XO being on the Bridge, isn't it wise to have your second in command nearby if the CO gets incapacitated?
What happens if both your CO and your XO are on the bridge together and the entire bridge is destroyed?
Chain of Command
This happened in one episode that put Troi in command of the Bridge. But if the Bridge was knocked out, command would fall to the next in line, which would probably be the CEO assuming that person was not on the bridge.
As far as XO's duties, I seem to remember an episode TNG called Second Chance that introduced Thomas Riker. Picard was telling Will Riker about his Duties as an XO but I can't remember what they were.
Just because the XO doesn't fulfill an extra duty on the ship like Chief Engineer or Chief Science Officer, doesn't mean he doesn't have other duties.
The other XOs on the list have other significant duties that they must perform.