It's not the controller. It's the Adapter(the thing that's being sold, and one controller, when you can buy multiples of), so everybody can easily play Smash Wii U on your system without any form of batteries. It's hardly the same thing alone. If it was a single player game, your point might be a bit stronger, but 4 players using one adapter is an entirely different story.
Just because you don't find it worth it doesn't mean some won't. Even for one game, it may be enough. And it doesn't matter how different they are. It's a matter of "is that worth it overall?" There is no right answer for that. There's just an opinion.