I'll throw in two additional benefits for a D800 as a wedding camera:
Weddings start usually during daytime so harsh lighting conditions might be something to cope with as well. So a high DR around the base ISO to hold details in the highlights is one benefit that it shares with the best DSLRs in this regard like the D4 or, to a lesser extend, a D7000.
Another unique advantage: Mount a fast prime and crop happy. This flexibility might compensate for some low light weeknesses.
And when you need the details for the formal shots under more controlled conditions you have them build in.
The only downside besides a lower frame rate at this point for me would be the PP slowdown when dealing with the massive file size of +1000 shots...