I think the Gamepad should have been an Accessory that came with Nintendo Land or ZombiU. I think after those two launch titles only Game & Wario used it in an essential way.
That is 3 games total since Wii U launched (eShop titles included) that use the Gamepad in an essential way. I equate the Gamepad to the Kinect which also doesn't have much example of essential use in games.
I think any Accessory that is not essential for wide variety of gaming uses should be designed to be optional. Nintendo and Microsoft are both wrong to have forced the inclusion. It is the wrong thinking that if many people have your accessory that somehow developers will get more comfortable and ambitious about developing fresh game designs and AAA games that use that new accessory.
It is not even just about the cost, it also makes the product less attractive to the comfortably core gamer. I think they worry about gimmick controls tacked on to their core games ruining the fun of those games or just shallow shovel-ware like the Wii got tons and tons of.
Bottom line: Core gamers don't like gimmick stuff because that stuff tends to collect a lot of dust aside from the few games that make essential use of that gimmick.
The gamepad isnt a gimmick though. It allows the designers to design games differently, there are gimmicky manners to do this and there are not so gimmicky ways of doing it.
For instance. Compare Uncharted on the Vita to Tearaway, Tearaway perfectly blends the VIta's hardware concept and the gameplay into an enrapturing experience. Uncharted and AC Liberation however are just gimmicky crap tech demos with clunky gameplay and full of imprecision. Its just a glorified showcase of what the system can do.
IMO Wii Sports was like that. But people played the shit outta it because it was fun and motion controls were a huge marketing novelty that is easily understood.
You see PS4? Its easily understood. Only people holding out on it are holding out for content.
With WiiU people are holding out for content and are not yet sold on its concept in entirety. That is NIntendo's problem