Even if the self is an illusion, what we call the self reflects a fundamental fact about the aggregate of all human consciousness: informational links are much thicker within a human being than between human beings. Even a Utilitarian social planner who has no doctrinal attachment to Libertarianism should take advantage of those dense informational links within a human being by allowing each person to make decisions about his or her own life.
John Stuart Mill makes that case in On Liberty, Chapter IV, “Of the Limits to the Authority of Society over the Individual” paragraph 4:
But neither one person, nor any number of persons, is warranted in saying to another human creature of ripe years, that he shall not do with his life for his own benefit what he chooses to do with it. He is the person most interested in his own well-being: the interest which any other person, except in cases of strong personal attachment, can have in it, is trifling, compared with that which he himself has; the interest which society has in him individually (except as to his conduct to others) is fractional, and altogether indirect: while, with respect to his own feelings and circumstances, the most ordinary man or woman has means of knowledge immeasurably surpassing those that can be possessed by any one else. The interference of society to overrule his judgment and purposes in what only regards himself, must be grounded on general presumptions; which may be altogether wrong, and even if right, are as likely as not to be misapplied to individual cases, by persons no better acquainted with the circumstances of such cases than those are who look at them merely from without. In this department, therefore, of human affairs, Individuality has its proper field of action.
John talks not just about an individual knowing more about his or her own situation but also about how the individual cares more about him or herself than others do. Letting people make decisions about their own lives does a lot to take care of bringing the the strongest preferences into social choice.
But in addition to simply making sure that all strong preferences are well represented in social choice, letting each individual make decisions about his or her own life makes sense also because each person also typically has a knowledge advantage not only with regard to circumstances, but also with regard to his or her own preferences. Without a great deal of tricky inference, one of the most difficult things for someone else to know about me without me telling them, is what I want and how much I want it. One of the most basic jobs of any adult is to carefully figure out what he or she wants. It is difficult for anyone else to do that for the individual, though software designers for websites like Amazon, Netflix, Pandora etc. are trying hard to be able to predict what someone will like.