Sentence:  Up to a certain point doctors, like carpenters and masons, must earn their living by doing the work that the public wants from them; and as it is not in the nature of things possible that such public want should be based on unmixed <WSD>disutility</WSD>, it may be admitted that doctors have their uses, real as well as imaginary. 
Meaning of the word marked with <WSD> tags:  uselessness, a lack of utility