To prime or not to prime, that is the question. Lots of modellers do, lots of modellers don't, each modeller has their own reasons for their method and as we only do this activity for ourselves then of course that's perfectly acceptable (unless you sell primer and state modellers should always prime, but then I can understand why they'd want to say that).
As for me, I very rarely prime as I don't see the need. The reasons are;
Paint. I use GSI Creos and Mr. Paint which both etch into the plastic so durability doesn't come into it.
Kit choice. I normally only build kits which fit very well and thus need little or no filler. Seams are easy to glue and clean up so I don't need to hide flaws. If flaws become apparent after the paint goes down, I'll fix it and touch up.
Detail. Some panel lines, rivets etc can be very fine so another paint layer will affect the depth and crispness of the detail. Some primers are very fine and won't affect this much, but they all will to some extent.
NMF. For piece of mind I will normally prime underneath Alclad (my paint of choice). It's so thin it will show every teeny tiny little flaw...
But...
I have used primers though. I tried that Stynylrez stuff on the MiG-29. It was impressive, pulling into the detail but even some of the finer panel lines on the nacelles were affected to some extent and the wash struggled. The model didn't need it, I thought I'd give it a go. I will also use it on the Shackleton I'm doing at the moment as there's been quite a bit of filling and sanding so it may help with getting a smooth base coat.
I do tend to use primers on my armour models, this is primarily owing to all the brass PE stuff I vomit all over them. It helps to unify the undercoat for an even finish and helps prevent chipping paint off the metal.
So that's my 2p on the subject. Prime everything, prime sometimes, never prime. It's up to you.
