Renfield47,
I'm starting to prime more and more. Normally I don't but after discovering the joy that is Tamiya primer in the rattle can, I find that I love using the stuff. Personally, I like to use it more as a filler of those last little scratches that I didn't smooth out the first time around. Also, if there are TINY gaps in the fuselage or wing-to-fuselage joint, it'll seal those up as well. Note that I said that those gaps have to be VERY slight. The primer is great stuff but it's not a magic cure all. LOL! I wish it was but, either way, it's still phenomenal stuff.
It goes on amazingly smooth! It's like buttah! I never used to prime any of my models but now I'm doing it more and more. It really does give the following coats of paint something nice to grab onto. My favorite so far is the Tamiya primer but I've also had no issues at all using the Model Master primer.
Eric