I honestly never played Splatoon without motion controls. That was more because I was too lazy to turn it off and it seemed pretty fun, so I kinda stuck with it. I mean, it doesn't really help with my superior lack of aiming abilities, but it does give me a few advantages over pure stick controls, most notably being able to "snap onto" targets really fast, which is the closest you can get to mouse + keyboard on a computer. There are a few videos that compare the two if you want to have some more elaborate explanation why motion controls are objectively superior as well. Please note that it doesn't mean you can't do well with sticks, but expect pretty much everyone in the highest competitive ranks to be motion control users.
One thing I'd like to add for Splatoon 2 - the Joycon gyro has been all kinds of iffy for me, and the controller's design made it easy for me to activate my specials on accident, or do other weird stunts that weren't intended or useful. The Pro Controller has a larger range of movement for the sticks and feels more precise, and honestly, a lot more ergonomic than the Joycons and I can only recommend upgrading to one eventually.