The problem with the utilitarian-consequentialist viewpoint, at least from a deontological viewpoint, is that it can lead to tyranny. For example, the first utilitarians, Jeremy Bentham and John Stuart Mill were advocates of liberal-libertarian economics and of a free society. However, rooted within their very philosophy was a 'Trojan horse' - if society can be shown to be more prosperous because of liberal economics, then excellent. But, if society can be shown to be more prosperous ('greatest happiness for greatest number') by allowing the State to manage all affairs, then so be it. The conservatives in England at the time were right for deploring Bentham and the Mills - the next generation of utilitarians were advocates of socialism.
Mises also understood this and that was the reason why he called Mill the great advocate of socialism, even greater than Marx, Engels, Saint-Simon, or anyone else.
If the only reason you like libertarian economics today is because you believe it is the most efficient way to allocate scarce resources, then what happens when some neo-Marxist comes out tomorrow with 'evidence' that the 'best' way to manage the markets is with some centralized interventionist scheme? While we deontological libertarians believe that free markets are the most efficient way to allocate resources, that is not the reason why we advocate them so; rather it is precisely because of our a priori philosophical beliefs regarding non-aggression, private property, etc.