Let us talk about the marginal product of worker i in occupation j. In most cases, this is indeed zero. My marginal product would be zero in fishing, medicine, and many other fields. In a complex economy, if you were to randomly assign workers to jobs, ZMP (zero marginal product) would be the norm, not the exception. The more complex the economy, the more carefully workers must be assigned in order to avoid ZMP.
Cue educators' and students' worries about portable skills and finding jobs that can found careers. Job selection's just going to get harder, so we'll hopefully see some innovation along these lines: perhaps a mapping of tested aptitudes to forecast skills demand? Essentially a mashup of US Occupational Outlook and aptitude/psych tests.