What other option is there, really? They can only put so many things on each controller piece. They could make both sides perfectly symmetrical, perhaps with something like the old Playstation style d-pad. I don't know, but I seriously doubt they're going to both putting a set of four face buttons on the left side when the d-pad achieves almost the same functionality.
Let me see... how can i explain.
Why is of such importance to hold each tiny piece NES style to the point of compromising the device to pursue either new interface solutions or practicality?
Edit: In simple terms leaning so much on the traditional button schemes is compromising the device functionality and innovation from my point of view.
Would be great if the mock ups at least did an effort to come up with a solution to this, would be a ton more useful i think.
Edit : And I think the mock-ups are pretty helpful, personally. Especially ChrisRo's, which shows what the detachable controllers could look like when detached. Helps sell the concept, imo.
But selling the concept is Nintendo's job. Our efforts should be concentrated on how it could work and what it allows. All of the mock ups are the exact same Eurogamer image, watching then haven't brought up any new insides on the device.
Most of them put the face buttons and Dpad in impractical positions when they are operated with one thumb (handheld mode) due to how apart they would need to be, just because they need to accomodate the NES style of play when detached (home console mode).