I think they're taking a multi-pronged approach.
Initially the Wii sold buckets based on the promise of motion controls, ease of use and immersion as well as going heavy on casual games. That got them an initial boost, but sales have been slowing, and they've shifted gears towards re-releasing classic titles that draw on nostalgia. A lot of Wii developers are now making use of the classic controller (pro) for there games, and a number of Wiimote games don't make much, if any, use of motion controls. So the Wii is still selling, but its original primary selling point isn't what's getting it anymore.
Nintendo I'm sure realises this and is deciding to sell it both based on 3D capabilities as well as classic titles.
I do have a number of concerns about the 3D though. First off, the 3DS has a gyro sensor, but 3D apparently only works when you're looking dead on, so you won't be able to use both together (rather like how you couldn't use Kinect with 3D shutter glasses). To me the tilt sensor is much more appealing for flight and racing games than the 3D, so it wouldn't be surprising to see 3DS games released that don't make use of 3D. Looking dead on is a further problem just in terms of not being able to relax. I've used a handheld keyboard before that could only be held in a certain position, and it got tiring.
The other thing that concerns me is what the devs were talking about with Kid Icarus. They were saying that having things move towards you in 3D caused strain, while having things move away worked better. So Kid Icarus where you're flying after things would work. Mario Kart would work quite well too. Metroid Prime though? Not so much. That limitation is going to be similar to the limitation of motion controls - where they actually stand out is in FPS games (Metroid Prime 3, RE4 Wii), rail shooters (Time Crisis 4, Sin & Punishment) and maybe a few other game types. You certainly can't base an entire library around 3D any more than you could around motion controls.
So Nintendo really needs to offer multiple reasons to buy the 3DS, especially when the classic reasons to buy a Nintendo handheld - price and battery life - are gone.