The only real reason is if you lack a WiiU, and want to lab on it. Like, you can still practice if you go to tournaments and friend's. I was there for well over a year, and it hurt my game a little (granted, I was going "over the top" with the demand on my tech skill). If you main, say, Bowser, chances are, you'll do perfectly fine (low-learning curve and playability).
Now, is it practical? Well, that depends. How's your 3DS fairing? What about your hands? are you actually flowing among the curve nicely, or is it limiting you? C-Stick makes a word's difference for characters that use turnaround F-Smash (in my case, Corrin).