Economic arguments, while important, are insufficient. Divorced from morality, economic arguments concede the moral high ground to those who wish to violate property rights. The result is essentially an attempt to compromise; it is an attempt to agree on moral principles while arguing for a different application of those principles.
In "The Anatomy of Compromise", Ayn Rand wrote:
In any conflict between two men (or two groups) who hold the same basic principles, it is the more consistent one who wins.
Consider conservatives, who argue for property rights on religious grounds. Property rights, they claim, are necessary so that we can prosper in order to serve God and do His work. The conservative argument for property rights rests on altruism.
A more secular version argues that property rights result in "the greatest good" for society. But, one's values ultimately determine the meaning of "good". Socialists argue that equality is the "greatest good"; the Taliban argue that the Sharia serves the "greatest good"; environmentalists argue that protecting the snail-darter serves the "greatest good". The mountains of evidence demonstrating that property rights lead to economic prosperity would be meaningless to these groups. They simply don't care about economic prosperity.
The utilitarian argument--"the greatest good"--also rests on altruism. If a particular action or policy will result in "the greatest good", but violates the rights of a small number of people, it is justified according to utilitarianism. The good of some must necessarily supersede the good of others. In the end, some will be forced to sacrifice for the "greatest good".
This is why utilitarian arguments are futile. They embrace the same altruistic morality as the enemies of property rights. As Rand identified, in any conflict between two men who hold the same basic principles, the more consistent will win. In any conflict between two altruists, the more consistent will win--the one who more consistently demands sacrifice will be more convincing. This is a part of the reason why Republicans continue to get stomped at the polls.
There may be some who are convinced by economic arguments. From my experience they too are altruists. But they embrace the flip side of the altruist coin--rather than sacrifice themselves to others, they sacrifice others to themselves. They agree that life requires sacrifice and victims, and they choose to be on the receiving end.
Any argument for property rights that is divorced from morality will ultimately prove ineffective. Such arguments attempt to ignore the purpose of rights, as well as their source. Rights are not a gift from God--as the conservatives claim--or a social convention--as the utilitarians imply. Rights are a requirement of human life--they sanction an individual's freedom of action in a social context. Rights allow an individual to act in accordance with his own rational judgment, in the pursuit of his own chosen values.
This is not to say that economic arguments have no purpose. They do, because the practical consequences of freedom are an important fact. But the practical is also the moral--the purpose of morality is to provide us with the principles necessary to live and prosper. Morality is the cause; a successful life is the effect. The defense of property rights must include both the cause and the effect.