I really really like this idea and I hope SE considers it.

As a solution to the idea for spectators swapping in for a d/c'd party member, just make it so that spectators get no loot in that scenario. You'd be helping the team finish the raid, thats it. This would be win for everyone really; as the party you get to finish the dungeon in full, and as the spectator you get something to do when you'd otherwise have had to just sit. If you don't like the idea of no reward for your work, then just continue spectating.

C'mon SE, the NA community never goes 8 pages without throwing poo at each other. This idea is obviously well loved all around. Chop chop to the drawing boards with it after ARR launch!