I don't think you can say you need to prime resin, but it can certainly be a good idea.
The surface of the casting and how much handling you need to do are big factors. But a lot depends on the type of paint as already mentioned: some paints, like enamels, are tough and form strong bonds with many surfaces; others, like *cough* most water-borne hobby paints *cough* aren't and don't.
If you degrease the surface of resin well you can get water-borne paints not to bead up when first applied, which can easily make you can think everything is going fine, but just a little handling (necessary handling, not holding the kit during painting) can show how wrong this impression is.
First-hand experience with paint wearing through on the high spots is the very thing that led many modellers to look for proper primers in the first place and to subsequently recommend them.
Einion