I actually find myself needing to use the 3D feature in order to play the games comfortably. The games I have, like Super Mario 3D Land, feature levels or areas that are unplayable without the 3D feature because of the need for depth perception. In the title screen, for example, viewing the tutorial level in 2D makes the platform appear to be an impossible shape, when there is actually a block underneath the top blocks, and the camera angle makes the blocks on the top block the empty space underneath it. With the 3D on, this platform is visible and you get a sense of how far away it is beause of the 3D feature.
Not only that, but the images in the 3D games just look sharper and more clear, and in some cases the graphics are improved when the 3D is on. I turn off the 3D when I play New Super Mario Bros. 2, though. It looks nice with the 3D on, but it's a huge battery eater.