Recently I've participated in (video'ed) a Primer class that included two very experienced divers who were both planning on moving forward to take Fundies. These two divers were both full cave and each had some mix training, as well as varying levels of exposure to DIR diving. Some people questioned at least one of the student's reasoning behind taking Primer. I didn't. Taking Primer stacked the deck in these divers' favor. The more they worked through prior to Fundies, the more they would learn IN Fundies. At some point during Fundamentals, even the most experienced divers can reach a saturation point. If some of the learning occurs prior to the Fundies class, then the student can go absorb that much more in the Fundamentals class - whether it is fine-tuning skills or learning brand new ones.
So, I heartily recommend Primer, unless someone has been actively mentored by individuals who definitely know what they are doing, or by the GUE community such as they have in the Pacific NW. The two I was referencing above did not NEED Primer by any means, but they both agreed they were glad they took it. For new divers, or "new to GUE" divers, the exposure to the propulsion techniques and buoyancy/trim concepts taught in Primer, allows for refinement of those skills in Fundies as well as bandwidth for new skill acquisition. Of course I would not go so far as to say it people can't have a great experience going straight to Fundies - look to this class report as an example! But I do think as a general rule, if it's possible from a time and money perspective, Primer should be considered, if at all possible. ... I do think a lot of people (more than is understood) take classes like Primer, Essentials, and Fundies to become better recreational divers. I know that was my motivation*.
