I've done a LOT of wet side plumbing... If it is going to have any sort of pressure you want to use it. The primer does a couple of things when using it. one, it cleans. two, it melts the PVC slightly to make it sticky and when it all evaporates the pieces are essentially welded together. three it gives the glue something textured to adhere to.
You don't have to over do it on the priming, but for pressurized pipes I deem it necessary. Drain pipes you can just glue so long as they are clean.
Good Luck