I would rather go for the interpretation of Immanuel Wallerstein who sees subsistence-related activities as a necessary compliment of capitalist development - in history, that is the 19th century, as in today's third-world-countries. So, if the "semi-subsistence" is more that a hobby (as "urban gardening" and so on), it could be linked closely to capitalist production and its flaws.
My comment questions the scope of your definition of semi-subsistence, which I think should also incude sub-optimal primary production units, although I sense the purpose of your definition relates to the private sphere rather than the commercial. My comment relates to family farms that opt for a 'semi-subsistence' form of farming in order to stay in farming and on their farm.
If you consider small-scale farmers who opt to persue their livlihood through alter-channels and by developing agri-tourism businesses, then yes, it's about control and desire. In the case of control over production, alter-channels provide opportunity to diversify into niche commodities, diversify production bases, and diversify consumer markets. Agri-tourism means they can capture the full value chain at the farm gate and horizontally diversify as well, delving into value-adding, and hospitality services. In this way they can retain an 'authentic' rural lifestyle, live an agri-culture, one that from a lifestyle perspective is more authentic than being an industrial farmer.
Here too, the reasons farmers make these choices is related, in part, to what Philipp Altmann has said on the question, that is, food production as something more than a hobby, and capitalist production and its flaws.
I have had a lot of exposure to people responding to fears of petroleum depletion by seeking out semi-subsistance rural lifestyles. They want to live by permaculture, cooperating in small communities. They are not interested in a profit-based economy and therefore do not have to maximize production - which tends to deplete soils and requires a lot of work. This is problematic in my country because the cost of rural land reflects a norm of profit over ecological sustainability and a forced competition with other land-uses that have much greater profits - such as real-estate development. Where it was once possible to sustain a family and community, sharing resources, now, because of policies to maintain massive urban populations, much higher 'productivity' is required and everything has become much more expensive and farms that were once economically viable are no longer. This economy does not make much sense and is not viable in the long term. I think the peak-oilers are right to head for the hills, but it is hard for them to do so with land-prices so inflated.
Lifestyle entrepreneurship has been frequently associated to the develpment of rural tourism units, either with or without agricultural production, to the "return to nature" movement, a search for a more healthy life and also a nostalgically embellished "rural idyll", with more personalized human contacts, associated to traditional communities, being at the basis of what Keith Halfacree discusses as "counterurbanization", also referred to as "neo-rural" movement of individuals discontent with urban life and choosing an idealized "rural lifestyle".
Lifestyle entrepreneurship in rural tourism has been discussed by Ateljevic and Doorne (2000) and Komppula (2004) as comprising social and cultural values as success factors along with economic benefits, with maximization of profit generally not being the main objective. Here, a certain type of food production as a statement of concern with health and econolgical sustainability, may be part of this lifestyle, thought by both rural tourism entrepreneurs and their clients.
It may be interesting to think about this question in light of opportunities and movements for increased urban agriculture in many developed countries. In the US, there is a "ruralization" in some parts, sometimes significant parts of so-called Rustbelt cities, Detroit being the most well-known example, but Pittsburgh and Cleveland also relevant. Nathan McClintock's 2010 paper, Why Farm the City? Theorizing Urban Agriculture through a Lens of Metabolic Rift is a good place for theoretical grounding on this phenomenon. The significance of "semi-subsistance" clearly differs in terms of class, ownership/control of land, individual or collective enterprise. It merits attention now in both rural and urban settings in developed countries.
About semi-subsistence farms the best expert is Prof. Sofia Davidova from Kent University. Please see her comments and publications here on the RG and in your CV