This is a very thought-provoking question because so much of it hinges on how you choose to define "useless" ...

that category of human activity that is not capable of producing or expressing any tangible sign of practical relevance and utility

that category of human activity that is so fundamental that it cannot be easily expressed in terms of language, and must therefore be 'translated' into more concrete examples before you can even begin to understand it, let alone discuss it

If you were a lawyer, someone could invade your country, invalidate all of your laws, nullify your statutes, repudiate your constitution and burn your flag.
A "lawyer" would consider herself out of a job.
A "Lawyer" would know that there is a fundamental pattern *behind* all statutes, codes and constitutions, and that no matter what system gets invoked, she would find her way within and around it, or even use that understanding to competently change professions.

If you were a software engineer, someone could switch technologies without asking you, or wipe the technological basis of your core competency off the earth with market momentum or patent lawsuits.
An "engineer" would consider himself out of a job.
An "Engineer" would know that there is a fundamental pattern *behind* all the platforms, languages, competing 'standards' and vendors, and use that knowledge to find a new niche, or even completely change professions.

Probably one of the biggest factors to determine whether or not something is "useless" goes along with the labels that you attach to it ... and to yourself.