Nintendo already did testing with multi-buttomed Wiimote prototypes. And let's not forget that the Wiimote is a gaming device, not a controld evice. There is no reaction component to changing TV channels, you don't lose the cable if you can't find a button. But the ergonomic strictures for a game device is much more stringent, which is why you see people already complaining about needing to use the D-Pad or minus buttons. More buttons simply wouldn't ergonomically work on the Wiimote, you'd have to be shifting your handhold, which is a big no-no if you want to attract the uninitiated to gaming. Just try walking around town in DQ Swords, ugh.
In fact, the Wiimote makes plenty of sense in it's current style. One button for your thumb, one button for your index finger, and just wave it around. Miyamoto's been moving towards this concept ever since the big GC button and his interest in one-button gaming as seen in Kirby's Air Ride. With the Wii, he may have finally gotten it right.
As for the DS, it's a chimaera. That's why it's got so much on it. But some of the most successful games on the DS, actually, completely ignore the buttons. Especially the ones that "expand the market," like Brain Age or Nintendogs. In fact, by holding the DS in the "book" position, Nintendo is basically hiding the buttons from casual players. Perhaps if Nintendo has a way of hiding the extra buttons on a Wiimote, that could've been a good solution. But the Wiimote as simplified-removable controller from larger shell idea was already considered by Nintendo, in fact, it was the first idea they considered and eventually moved away from. If I recall, actually, the idea for the nunchuck as a seperate control unit (hence causing the move away from unified controller with detachable wiimote) came from a western developer: Retro.