First off and how we know, Just Dance 2017 is a confirmed launch title for the Switch and guess what's necessary to play Just Dance? Motion controls (or something like kinect, but that's very unlikely). Why would they even announce Just dance for a console if there's absolutely no way to play it?
Secondly, motion controls allow certain Wii U games to be ported to the switch that used motion controls (example: doing a trick when going off a ramp in Mario Kart)
Thirdly, I see no real reason not to keep motion controls as getting rid of them decreases the amount of possible games the Switch could have.
And fourthly, the patents mention the Joy-Cons having motion controls.
Edit: I would like to clarify that I made this post because at multiple occasions, I've seen people deny the fact that the Switch will have motion controls.
Discussion started at here by Cannibal_Turtle