Shimano does indeed say their 6700 and 5700 10-speed chains are for "Double Chain rings Only" and no one, even Shimano's tech guys, seems to know why. When I wanted to buy a replacement chain for my Ultegra triple crank, I considered a 5700 until I saw the disclaimer. I called Shimano USA's tech line to ask why and the tech guy really couldn't give a reason for the limitation and wasn't sure what would happen if you ignored it. I wound up getting a 6600 chain which has worked well.

Since then I've seen a couple of postings on this forum from riders who have successfully used 5700 or 6700 chains on triple cranks so I still don't know what's the problem.