Motion controls are not necessary in order to get to S, I've done it without them, but I recently picked them up and boy oh boy, do they help. At first, you feel like giving up and going straight back to sticks, but trust me, if you practice for a good week or two only using motion controls, you will never go back to using the analogue stick, of course the stick will still retain its uses in tandem with motion controls, but it will not be the main source of camera movement, if you do get accustomed to motion controls, going back to the analogue stick will feel extremely limiting and rigid. Whatever the case may be, you should definitely give them a shot if you want to improve your gameplay overall, you'll find that a lot of S+ players use motion controls, as well as almost all of the prominent and competent members of the splatoon community like fiveleafclover, thatsrb2dude, hitzelburger, keykey, just to name a few.