Several things come to mind.
1. Nintendo want the controllers removable in order for the NX to stand as isolated 8" tablet (I suspect well see more of this next year)
2. Convenient local multiplayer on the go+ at home. Out the box you get 2 controllers and Instead of taking full size gamepad out with you the sticks are infinitely more ergonomic. In either case the biggest games (Mario Kart, 2D Mario, Smash) will work quite well with them.
3. motion controls. Nintendo is keeping this hush for now but Eurogamer and Emily Rodgers both reported full on motion controls.
All in all, the detachable controls is just a more ambitious design and allows for a more exciting device. If you end up liking the games that comes to the Switch, a $50 pro controller isn't that steep of an addition. I mean who didn't buy seperate controls for Smash Bros Brawl?
As far as Nintendo's cost are concerned, packaging a built in controller and a seperate one for home console use, would be just as expensive as what they've opted for







