Here's my take. Suppose you have a piece of land which is completely unused: nobody lives or visits there, nothing is being built there, nor is the land held with the explicit intent of being left in its natural state. In that case, since the value of the land is being wasted, there is some merit to the idea that it should morally be usable by other people (although keep in mind that the current owner might want to do something with it in the future). And in fact, if the owner is negligent enough, adverse possession allows someone else to do so in many jurisdictions. There is also common land and literal tragedies of the commons...

In all other cases, someone is getting some use out of the land, and in general allowing anyone else to use the same land would impair that use. (In some special cases, there are again laws on the matter: in most US states it is illegal to hunt on others' private property, but only if the owner posted a sign or applied specially colored paint. If not, since the hunting would likely have minimal impact on rural property, it is allowed.) Perhaps, according to someone's standard, the owner is making inefficient or wasteful use of the land; perhaps they have barely changed it from its natural state, while a developer would like to add it to their adjoining property full of valuable businesses or whatnot. But this is subjective, and doesn't change the fact that the 'better' uses would deprive the current owner of something - depending on the owner's sentimental attachment to the land, potentially something subjectively irreplaceable.

I do not have enough knowledge to directly argue in favor of capitalism. There are plenty of theories under which to posit that the current owner should be deprived anyway in the interest of the common good. But at least it is clear that our current system has certain significant benefits.

To take the argument to its obvious conclusion, for copyrighted works, the public domain would at most deprive someone of money they could potentially be making, which is hardly the same thing. Meanwhile, it would allow many different people to enjoy passive use of, or build interesting things on, the same property without getting in each other's way. The flipside is of course the standard justification for copyright: the prospect of that money encourages people to be creative, and getting that money allows them to make a living through creativity. But this does not mean copyright has to last forever. (It also doesn't inherently imply copyright should a right of exclusive control; in the special case of radio broadcasts of songs, making it purely a mandatory royalty, open to all by law, seems to have worked out pretty well. And then you get to fair use, but of course there are far more detailed arguments to be found there.)

But you know all that. In another post, you said:

> While that's one philosophical justification for property, we've had property long before anyone coined the phrases "rivalrous" or "excludable" and it has been justified for reasons that have nothing to do with those economic concepts: http://en.wikipedia.org/wiki/Labor_theory_of_property. Going back further, see Psalm 128:2 ("For thou shalt eat the labour of thine hands: happy shalt thou be, and it shall be well with thee.")

For one thing, I don't think coining the phrases "rivalrous" or "excludable" means much. If you're making a theory of property, where property is defined as actual physical objects or land, you don't need to make up terms to describe what obviously applies to all property; it's only when you start calling data property that you have to distinguish it from the rest. Any older philosophical argument in favor of property, regardless of whether it explicitly addresses the matter, was written, and should be read, with some broad ideas in mind that do not apply to data. This certainly applies to the Psalm you cited.

But actually, Locke makes it quite explicit - right near the beginning of the chapter "Of Property" in his Second Treatise on Civil Government:

And tho' all the fruits it naturally produces, and beasts it feeds, belong to mankind in common, as they are produced by the spontaneous hand of nature; and no body has originally a private dominion, exclusive of the rest of mankind, in any of them, as they are thus in their natural state: yet being given for the use of men, there must of necessity be a means to appropriate them some way or other, before they can be of any use, or at all beneficial to any particular man. The fruit, or venison, which nourishes the wild Indian, who knows no enclosure, and is still a tenant in common, must be his, and so his, i.e. a part of him, that another can no longer have any right to it, before it can do him any good for the support of his life.

This is just stating the concept of "rivalrous" without using the word! And it clearly does not apply to intellectual property at all; when a basic premise of his argument is voided, we should be hesitant to apply his conclusions.

edit: There is also the matter that as eager as philosophy is to be concluded from first principles, in general I think empiricism is superior. (And don't confuse 'common wisdom' which at least somewhat reflects centuries of empiricism with first principles.) Capitalism has its moral arguments, communism has its moral arguments, and neither are necessarily wrong; in my drastically oversimplified view, in practice it seems like pure communism doesn't work out very well, but socialist economics tries to satisfy some of the moral precepts of both in a more achievable way. But we haven't had centuries of empiricism when it comes to intellectual property - not that long a history of protection nearly as strong as today, and certainly not of the fundamentally new practical properties of digital intellectual works.