From a Darwinian approach, all of them would be useful. Without a variety of functions, it would decrease the overall survival of the population. You never know when each function might come to good use.
“Thoughts are the shadows of our feelings -- always darker, emptier and simpler.”
― Friedrich Nietzsche
What are your opinions? Would the world fall apart, if for example, Fe became extinct, or there was a global ban on Ni?
Personally, no, when it all comes down to the bigger picture. If you use the functions in the area and manner they were designed for, they can be extremely effective. A function only becomes problematic when the individual demands it to perform tasks beyond their scope. We can't have strengths without weaknesses (i.e the "good" does not exist without the "bad") and I think there's more you can do with a team of varied talents/faults, than a team who's 'well-rounded' in every department. The latter promises consistency, but the former can develop extraordinary products if they collaborate on a higher synergistic level.
If one function fails in delivering the results, then use another function as the support crew, or the leader. Maybe the first guy has a really great idea, and he only needed someone who's grounded enough to bring that into a reality. Together, the dreamer and pragmatist showcase their best work, leading to a unique project that neither could achieve by themselves.
"I cannot expect even my own art to provide all of the answers -- only to hope it keeps asking the right questions." -- Grace Hartigan