I hadn't heard this, and this is a big downer for me. I pretty much don't want to play any Wii U game that requires always using the gamepad. I know as with many things this may be a minority opinion, but I'd much rather use the pro controller instead and lose a little functionality.
If multiplayer online is going to require the gamepad and no other options, it's personally to the point where I would decide not buy the game, barring a patch.
*edit* Speaking of the gamepad, as always the battery dies when I leave it unplugged so I had to plug it in when I turned on the Wii U today.