I've never understood why we weren't held to the same standards with PVC as plumbers where primer is required. The only difference we have is that we are trying to keep water from getting INTO the pipe.
I've always used primer, and yes it has been the purple stuff but I'm careful with it where it is exposed so that it looks OK. On underground couplings, I really don't care about being neat with it. Still, it makes sense that the joint should be cleaned prior to gluing.
My experience has been that many joints that pull apart are due to too much glue. It tends to "over-melt" the surfaces and the hardened glue ends up being the only thing holding it together.
