The 3DS Slide Pad attachment is nothing like Motion Plus. It's not even close. First, Motion Plus made the Wii remote longer. The slide pad accessory turns the 3DS into a hulking Megazord. Second, 3DS no longer comfortably fits into pockets, making it not portable anymore. The same problem is not encountered on the Wii with or without Motion Plus since it's a home console and people don't normally carry controllers around. Third and perhaps more importantly, the slide pad peripheral isn't just adding functionality, it's patching a severe design oversight. I believe Nintendo claimed Motion Plus was too costly to implement into the original Wii remote. Fair enough. I think most of us bought that reasoning. With 3DS, Nintendo is charging people $10 for what would have cost them less than a dollar per unit and without the hassle had it been implemented in the first place. Everyone would have been happier.
I'm not opposed to the peripheral. I agree that it addresses a major shortcoming. However, I won't sit here and act like it's not a big deal when the implications couldn't be worse for consumers. It sets a bad precedent.
Nintendo made some of their best games without a second analog stick and they did so by coming up with ways around it. PSP developers did the same and games like God of War: Chains of Olympus were still fun. Nintendo didn't need to create the slide pad accessory. Some games will benefit but there's no denying that this makes Nintendo look bad. The missing slide pad was one of the first things everyone noticed and Nintendo had months to fix it. This makes their research and development department look like bumbling fools, especially since their solution looks like the way it does.