>>Would my pictures be demonstrably and measurably better if I shot full frame with only primes? I seriously doubt it.
Next up: why you don't have to stick to primes. Yet another thing that used to be true but mostly isn't now.
That said, the attraction of FF is that it's easier to go wiiide. I sometimes use 14mm on FF.I find super wide compositionaly hard so it's mostly something I plan to crop to a panorama (stitching waves is a PITA), playing with perspective, or night sky - which brings me to the other reason for FF: low light. It's still a dance between speed, dof, and iso - and sometimes there just isn't a way to get a perfect shot (sharp, no blur, all subjects in dof, brighter elements not blown out) but it gives me a better chance.