However, one can say someone may be on the wrong path to the common purpose of martial arts, if they cannot defend themselves against opponents and threats.
An expert martial artist might get killed in a bar fight; that doesn't mean his martial arts have not served their purpose.
I'd also argue that purpose and "use" are two entirely separate things. Iow, a martial art is a skill; how a martial artist uses that skill is up to them. Everyone who studies fighting could be considered a martial artist. Imo, the difference lies in everything the practitioner does that isn't connected to fighting -like health, but also mental (and spiritual) cultivation and character development. I also think why a person uses their martial training to fight is more important than whether they can fight well, or even win.
In fact, I'd ask a potential student why they wanted to get to Rome in the first place. Some might want to go into the UFC or K1. Others might say they just want to get stronger and healthier. That will determine their paths. People study/train the same martial art for totally different reasons.