According to manufacturers, primers and paints have very different jobs. Primers are designed to adhere to the substrate and create a uniform surface for the top coat. Primers penetrate and adhere to raw surfaces, evening up porosity so that the final paint film doesn't look blotchy. Primers for wood, whether they are latex or oil-based, contain specialized ingredients that top coats don't necessarily have, or have in lesser amounts. Stain-inhibiting tannin blockers or preservatives that are found in primers, for instance, are designed to soak into raw wood but not into finished surfaces. Primers for other surfaces have similarly specialized ingredients.
Paints, on the other hand, have an entirely different job. They add color, sheen and surface protection. As a result, paints are likely to be made from different resins and contain different additives than primers.
Ian Freer, a customer-service technician for Canadian-based General Paints, says it's important to choose the primer best suited to the surface you're coating. Latex primers, for instance, are the best choice for unfinished drywall, he says, because an oil-based primer can make the top paper layer fuzzy. On raw interior wood, oil-based primers may be a better choice because of their surface penetration. Outside, latex primer lasts longer because it's more flexible. A good paint store, or a manufacturer's technical rep, should be able to answer specific questions.
When no one's looking, you can apply paint without benefit of a primer. But the finish top coat is unlikely to last as long or look as good.