The libertarian position on liberty as the highest good makes sense if one views humanity according to the libertarian belief that man is a rational, economic animal who acts in his own interest. From that position, people who make bad choices merely need to be corrected when they harm others, and more extreme cases need to be imprisoned for their crimes. But what if man is not rational, but rationalizing? What if man frequently acts against his own interest in truly absurd ways? Can anyone honestly say that from a cost-benefit analysis that regular consumption of most hard drugs makes sense? Given the rampancy of sexual diseases, especially severely damaging and deadly ones, are promiscuity and prostitution rational rather than rutting russian routlette? These are just the tip of the iceberg of the myriad ways in which man proves that he is hardly a being who subordinates himself to raw reason rather than employing reason like any other tool to get what he wants.
Even as an "economic animal," ordinary man is hardly rational. Would any rational being get themselves into several thousand dollars of high interest credit card debt in ordinary situations? It is certainly rational for someone to bankrupt themselves with credit card debt to finance medical operations, but consumer electronics, dining out and other luxuries? Hardly! The entire consumerist culture which encourages people to spend beyond their means, to save little and that despises frugality is irrational, to say nothing of the economic intellectual support behind it.
Reason is nothing more than a tool, and as such, man can no more live by it than he can live by fire, by hammers, by computers or any other tool with which he chooses to enhance his life and surroundings. Being an intellectual tool, he can employ it to the point of perceiving that he is using it to such an extent that it guides his every actions, but that's nothing more than intellectual conceit for it does not guide him so much as exist as a habit. The greatest danger that comes with this fetishization of reason is that it ignores the way that pre-rational desires and inputs affect the rules of logic and thus serve to create subtle blinders. Fetishizing reason has the peculiar habit of ultimately creating the same sort of haughty arrogance and disdain for disagreement that one finds in religious fanatics.
Once it is accepted that man is not truly rational (which is not the same as calling man irrational), and that he often acts against his own interest, there is an instinctive impulse to save him from himself. This is where many Christians go wrong, for man is perfectly capable of choosing right from wrong, but chooses not to in many cases. It is that facet, the choosing to do evil, which is what vexes Christians and which the Christian framework says cannot be changed by force. Preventing someone from getting high will not make them not want to get high (which is, according to Christianity, where the sin is rooted) unless the state is so successful at eradicating intoxication that man simply doesn't know that it is biochemically possible. Such a state would have its own extreme externalities, the very least of which would be wanton abuses of authority.
The trouble that Christians often find themselves in with reason is that they often play up the power of emotion and desire in overpowering reason to the point of making excuses for why someone harmed another. Libertarians (of the secular, cultural sort), likewise, often lack a proper appreciation for the ways that emotion and desire can so thoroughly rob people of reason, principle, etc. that they are closer at times to animals than the sort of hyper-rational being that mainstream libertarianism assumes man may be. A strong example of this is the way that many ambitious people will gladly arm hostile, tyrannical governments with the means to oppress and murder so long as they reap profit and personal luxury. Libertarians will often defend this saying that it is in "their rational, self-interest," but it is ultimately in no one's rational, self-interest to provide ideological governments, the greatest killing machines in human history (as decisively proved in the 20th century), with the means to efficiently carry out their will. That makes no more rational sense than selling weapons to known criminals who live within walking distance of your neighborhood.
Where the two may come together is by accepting the limits of human nature, especially as they relate to emotion and reason, and accept the fact that these limits have little bearing on the question of personal responsibility. A biological tendency or character flaw does not excuse bad, even evil, behavior. Likewise, a culture without limits on human autonomy and which provides no guidance to desire and emotion will feed into the worst traits of humanity by giving them a continuous outlet; if man is to live with reason as a guiding tool, it needs the assistance of a culture which helps his reason control his lower instincts, his emotions and desires. Therefore, as an example, a free society may legally permit him to be promiscuous, but would not socially permit it. The state may not care a whit about how many women he beds, but his male peers would be free to exclude him from business, from church, from every social institution. If nothing in a society controls this behavior, then the behavior will be accepted, and thus there will be more of it, eventually to the point of obsession as it currently is in America where there is neither cultural nor legal restraint and it is everywhere.