It depends on what you want out of it really doesn't it? If you want to really impress people with you're martial art's skills you need to A: Have a black belt. B: Be able to do crazy spinning jump kicks. or C: Get into and win a street fight (unfortunately C is something far to many "martial artists" go for).
But quite frankly you don't NEED that as a martial artist.
However it could be good to go up to black belt level in ONE martial art, to have a proper specialization I guess, but then I'm no expert on the subject.