Why practice a Martial Art if it's not useful?

In my opinion, all Martial Art styles are useful. Everything from Jujitsu, Taekwondo, Karate, Wing Tsun...whatever. They are all effective in my opinion if done properly. However, this one issue has been getting under my skin. It's that many times I will read or hear someone say when answering the nonsense question: "Which Martial Art is best?" an individual will respond saying: "I have a 2nd black belt in X but it's only good for fitness...I'd go with Y". My question is this: Why dedicate your time to something you find inferior to something else? If it's about fitness, why wouldn't you be any less fit practicing X Martial Art vs Y Martial Art? I'm just curious as to what anyone else thinks to this question...