Priming is not entirely necessary.
I have had better success with sanding then painting vs. sanding/priming/painting or priming/painting. The enemy of any transformable model's paintjob is friction. "Where the parts rub the paint will scrub." Primer adds thickness to the overall paintjob and rarely matches the top color so when the surface paint rubs away you see the primer underneath.
I tend to sand as much as possible then paint. After I transform the thing numerous times paint wear spots will show. I remove the part, re-sand, re-paint. Additional chips or wear marks I dab with a paintbrush as necessary.
On my primed pieces I do this much more often than on those I simply paint.
To each his own but experimentation will probably tell you what works best.
Sandable primer is great for fixing surface imperfections, and when you have a beat up part sometimes it is your only option. But as a rule I do not prime anything, and my paint seems to hold up amazingly well, especially DUPLI-COLOR's stuff.