Are you serious? First off, I'm a gamer and I don't own a tablet/smartphone, and I know others who don't.
Second, comparing a tablet to the Wii U gamepad is pretty silly, as whatever the tablet does, it'll never be as capable as the gamepad.
Finally, the gamepad is guaranteed to be used/owned by every Wii U owner. The phone/tablet isn't, so game developers will most likely never do anything with your tablet/phone, unless Microsoft nudges them with a wad of cash.
It's the difference between the XBox having the Kinect actually being built in, and it being sold as an add on.
Also, without the controller, the console would lose something.
Uniqueness, it's the only console of this gen that actually tries to be a different feeling console, rather then being a PS4/XBox One with a Nintendo logo slapped on.
And no, copying the Wii U half-assed doesn't make the Wii U itself less unique, it just puts into perspective how... unoriginal Microsoft and Sony are when it comes to their systems.