Keep in mind that a PVC joint is actually a solvent welded joint...the materials melt and when the solvent evaporates, they literally become one. The primer does several things: cleans any contaminats from the surface and starts to melt the surface and prepares it to be further disolved by the cement. The cement, by itself, may not penetrate the surface layer to make a good joint without the primer getting it started.
So, yes, you do need both the primer and the cement to make a good joint. In many places, the inspectors look for the characteristic purple color of the primer or they'll fail the installation. They do make clear primer, but it's not good in areas where the inspector is looking for the purple stain! (it is a lot neater, as a drip doesn't stain things, though).