There are seven reasons why I think that a collectively shared agriculture is a good idea. First, if we accept that a local economy is something to strive for (see Wendell Berry, Vandana Shiva, E.F Schumacher and myself among many for arguments pro a local economy) then there are still a few possibilities as to how to do it. There are those who believe that we should all follow our own bliss and everything will work out. That is, if everyone just does what they like and what they are good at, everyone's needs will be met. This is based on a myth that has taken over the imagination of most in this culture. It is a beautiful myth and it makes people feel good, but it is only partially true. It works for a few privileged people in the heart of the empire (but not most), and not for most of the world. The more cheap energy and resources are available to the west, the more "bliss work", i.e. luxury service work can be supported. But most people have to do non-bliss work, both in the west and the third world in the present global economy (the few people whose bliss is to farm and build are exceptions to the general rule that most people's bliss work is not hard physical labor, though hard physical labor can be fun for all in moderation and/or when shared). If energy and resources dwindle in supply, this will be even more true. And similarly, in a local economy that is not subsidized by exploitation of other parts of the world or theft from the future (in terms of non-renewable resources and debt that needs to be repaid in real energy), there is a significant amount of energy that needs to be spent on basic needs. Dividing that work roughly equally amongst all able-bodied people makes it more equitable. In this basic needs economy, there is room for some specialization, so not everyone has to make clothes, build make/maintain tools and do healthcare, but since the majority of labor will be agriculture, most people will have to do some agriculture to achieve an equitable number of hours worked.
Second, a gift economy for basic needs is unstable. It assumes that people who are toiling hard to provide basic needs are going to provide them for everyone out of love all the time. Those who are providing luxuries and not necessities are in a precarious position, dependent on the constant good will of the others to eat and to have shelter. Much better to have a less utopian vision of human nature and have everyone responsible for their basic needs collectively. That way, a gift economy for the "spice of life" can be on a firm foundation. People can follow their bliss knowing that with a bit of work on the foundation, they will be fed, sheltered, clothed, educated, and cared for when sick, old or very young. This is where specialization, trade and diversity of avocations and wants can flourish.
Third, working together on something that everyone needs and most can do, is more fun than working alone, and brings people together providing a basis for local culture.
Fourth, if one can produce one's basic needs oneself, one is harder to control. In the present culture, it is not the farmers who control people, but the market/corporations/media and a few wealthy folks who own the means of production. In a local economy where the farmers produce food for everyone and the builders and handypersons build and maintain shelter for everyone, it is they who accumulate power and can ultimately control everyone else. This is already starting to happen at Earthaven, where the farmers are getting private leases, except that the global food market is still competing with them so their power is still limited. To ensure freedom for individuals, they must be co-owners of the means of production, and not leave it to proxies, whether they are corporations, governments, wealthy landowners and industrialists, or farmers.
Fifth, the "follow your own bliss" myth is ultimately too egotistical. There is spiritual value in doing something that may not be your favorite thing, but that is helping others and needs to be done.
Sixth, shared agricultural work levels class distinctions that arise when some people do the hard manual labor and others do only intellectual work and easy manual labor.
Seventh, a shared agriculture, just like shared shelter and kitchen does with heating, storage, washing and materials, creates an efficiency of shared tools and if there are enough people, a labor pool able to respond to peak labor times (such as digging and harvest of field crops).
Note that only the third, fifth and seventh reasons argue for collectivization, whereas the other ones leave open the possibility of individual/family homesteads. There are some disadvantages to collectivization such as difficulty to coordinate people that might be traded off for these reasons (3 5 and 7).