It's not guerrilla gardening, it's more like guerrilla picnicking. A few summers ago, a group in Copenhagen, Denmark, took over a wild and abandoned urban plot full of wild berries, apple trees, and weeds. They created a public garden for the staging of a series of picnics. Their intent was to go back to nature within the city, and escape the commercialization of the urban environment.
From an essay on Free Soil:
The early allotment gardens in Denmark were connected to the workers movement and thus had a political component. Many workers got access to a plot of land away from overpopulated cities and small apartments. Access to land was especially 100 years ago a deeply democratic spatial project, as well as about having places for leisure and retreat from working life. Today it is harder to spot the project in the allotments and it seems there is a need to define what are the spatial rights worth fighting for now?Public Picnic's motivation is more idealistic and political than the Garden Posse, but we share the intent of reclaiming public space for communities, and valuing the natural productivity of the land. There are lots of places in Austin (such as the area around Quilombo) which are similar to Public Picnic's Copenhagen plot. Check out more of the project on their website: Publik Picnic.
An attempted answer could be that we need spaces free from the omnipresent regime of disciplining by media and commerce and institutions telling us how to live our lives, how to look, act and feel. Places where the collective is possible, where there is no division between production and play, between pleasure and politics; places for the life-world.
....This is the spaces we want, spaces in out neighbourhood where we can meet, organize and express ourselves in informal ways. It is a modest demand: inclusive spaces free of commercials, self organized urban free spaces where we live, everywhere.