The problem with the idea of the base station giving the NX more power when docked than when portable is that it essentially splits it into two systems for game development purposes. So any game made for the NX would be a cross platform game targeting both the mobile and the desktop NX (unless it's allowed to create games that cannot be played mobile?).
You can imagine what that means: Games need to be made with either two different graphics settings, target the lowest common denominator (i.e. ignore all the extra power that the base gives) or just stutter when played on the go (the Hyrule Warriors Legends solution to supporting two platforms of different power). And the difference would be huge if a portable-only version was cheaper than the dual purpose one, after all removing a docking port while adding a battery and screen doesn't exactly lower costs. It might be possible to offer a cheaper home-only version that lacks the screen, battery and whatever else a portable might need, like the Vita TV.
A smaller screen (lower resolution) doesn't save THAT much power, especially the CPU would have to remain the same or the gameplay would need to change between the home and mobile modes and the GPU couldn't save much beyond fillrate. You'd need to make a second, lower spec set of graphics (admittedly cheaper than making a second game but still very, very expensive) to really use both modes fully and most cross-platform games don't bother with that.