Sony originally locked the full speed of the PSP CPU at 266 MHz before unlocking the full 333 MHz like two years after release with a firmware update. I don't think Sony ever officially explained why they locked the CPU, but most speculated it was due to battery life concerns.
There was also a rumor (I don't remember if it was confirmed) that Nintendo did something similar with 3DS and unlocked a second CPU for 25% more processing. I'd imagine battery life being a concern here as well. I believe this rumor also mentioned something about Nintendo figuring out a way of handling stereoscopic 3D which would reduce the strain on the CPU for more processing power (of course, not using 3D entirely would help out even more).
If this additional core thing is true, the battery life explanation doesn't really work here. I guess the dev kit reasoning works. Even so, I doubt it would open up Wii U to Super Saiyan levels of power like people are making it seem.
Anyway, Wii U is underpowered compared to PS4/One. However, I'm pushing the shenanigans broom on anyone acting like good games can't still be made on it. The only thing many of these companies are improving is the graphics. They could use that extra processing on better AI and whatnot, but they're probably going right for graphics because they learned nothing about the last seven years.
I wish Sony and Microsoft weren't launching successors later this year. Or if they insist, sell hardware at a minimal loss. No one would feel the pressure to take advantage of processing power that isn't available. It's not good for the industry to keep pushing expensive technology when even the largest publishers are struggling to keep up with it. As consumers, we don't see the effects of these losses until companies close shop and we just won't get games from them anymore. Why would anyone want that?
By the way, the GamePad is a gimmick. That's not inherently bad. People just often read it that way. I love the GamePad though only games that can use it well should use it at all. Just because something is there doesn't mean it has to be forced into every game.