It is useful in curing alcoholism. Some users don't like the fact that it's illegal but they do not want it to be freely available. They want it to be regulated by therapists. LSD is extremely potent, such that the thought of it being available at the corner quik-mart is unthinkable. It is better to let people endanger themselves and others to some large degree, rather than attempting to have big brother protect us, and rob us of self-responsibility and freedom.
Life is dangerous.
Freedom is dangerous. We should not neurotically try to eliminate this danger by eliminating freedom. However, LSD is dangerous. I have a philosophical hypothesis that the worst danger of LSD is the danger of having a divine insight about the shaky foundation of morality, and feeling forced, in the name of Truth About Morality and Self-Control, to wield the sacrificial knife on an innocent person. The theory of religious sacrifice is an interesting area of current philosophical research.