Squatter movements challange our assumptions about property. If you think property is only a pragmatic right that we create to aid productivity (rather than a natural one) then, just as unproductive workers can be sacked, perhaps unproductive non-entrepreneurial owners should equivalently be "sacked" from ownership.
NaomiKlein on the Argentinian war against piqueteros. (http://www.thenation.com/doc.mhtml?i=20030331&s=klein"))
Piqueteros, the poor disenfranchised by Argentina's economic collapse, squat not only unproductive land and empty buildings, but also derelict factories, putting them back to work.
A supporter of HernandoDeSoto would try to legitimize their PropertyRights over this squatted territory as soon as possible. The existing government, apparently encouraged by the IMF, uphold the existing property rights (often violently) despite the lack of economic activity.
A big problem that can occur is that owners can suddenly decide to re-activate their holdings (change in market demand, etc.), and want to reclaim their holdings. That's when things get ugly. –BillSeitz
For more discussion of property and squatters, you might want to visit I'm http://squattercity.blogspot.com. I'm a writer who spent two years living in squatter communities across the developing world (Brazil, Kenya, India, Turkey) and have just published a book of reportage about them. –RobertNeuwirth