I said is willing to pay for - not necessarily that they can pay for it. Any one-sentence definition of a word as complex as useful is going to necessarily be incomplete, but I certainly mean to include externalities in it. If, for example, people value "a sense of belonging to a community" and are willing to give up something meaningful for it, but co-ordination problems or whatever else means it can't be captured by market forces, then I would absolutely view someone who creates "a sense of belonging to a community" as useful - provided that the cost of their doing so is less than the price that the community members would be hypothetically willing to pay.