Roger, I would agree but like DaveS53 said the polyester primers, particularly evercoats featherfill & Slicksand are basically really filler in a spray form, you have to even add the catalyst just like filler, if it settles along time its a solid chunk of talc at the bottom and must be shaken (no way of stirring even with a drill) the nice thing is polyester and fiberglass are the same base and totally compatible.
We all know there is as many techniques with painting as there are painters, I would say its by preference only as the system I follow was taught to me was by a custom painter and his paint jobs are what I consider perfect even up close. He applies filler first (Rage) followed by Slick Sand or Featherfill , blocked then next Epoxy primer to hold all previous work together, blocked and then sealer then paint, all body work is guide coated then blocked. I haven't really seen any of his paint jobs fail even over time and he has done many magazine cover cars as well as some of the most iconic rods built. The one problem with polyester is its not water proof (no wet sanding) and its real dusty.
I have to say I am know expert I simply follow what was taught to me by one, I am sure if you find a great painter and follow their techniques a great paint job can be obtained no matter what the process, more than one way to skin a cat.
I try to beware of some collision techniques, quite simply daily drivers are only kept for about 5 years before being sold so literally they can fill over rust and not have to worry about out it returning, i repainted a '69 camaro whose paint job was almost 15 years old (garaged) and found filler over rust, it took that amount of time for it to surface and the owner wasn't happy after we cut the small bubble with a wire wheel and found the mess, I think the big trick for me is getting it as close to perfect before ever applying filler and learning how to color sand & buff, but as I have said I am no expert!