For some people.
For others it is detrimental.
It doesn't simply "scale with SLI very, very well."
http://us.battle.net/wow/en/forum/topic/19644934297#2
Originally Posted by Blizzard Entertainment
The game takes no advantage of SLI or Crossfire configurations. It will tolerate them but occasionally we'll ask to have them disabled if an issue arises that we feel might be related to them, but's that's usually driver-related, and not always something the game is doing.
It is not specifically trying to utilise it, so your mileage may vary a lot.
I see other people saying that it actually makes things worse.
So until there is official support which has consistent results behind it, someone should not be aiming for a multiple GPU setup for WoW.
Do so for other games if you are in that position, and consider it a bonus if it works.