Mine are all pinned.
On a KAC IWS lower, if the backstrap were to drop down, you lose your takedown pin spring & detent(s).
once you’re sure which backstrap you prefer, and you don’t plan on switching out backstraps for different shooters, I see absolutely no reason not to pin it.
When the Magpul trials MIAD grips were shipped back in '04-05, I was shorted a roll pin with my kit.
Since I didn’t have the right size rollie around, I used a 027 track pin…and it worked perfectly. I pin
all my MIADS as a matter of course, since I use the same front and back for all of 'em. Never had a
core fall out, but then I use inner tube wrap which goes around the bottom to a degree. Good grip,
and keeps everything snug. Still the best AR grip on the planet!!!
Not pinning leads to a large amount of Self Loathing when realize that the grip only comes with one of your preferred sized backstraps and it is on the side of a mountain one helicopter ride away. Not that I would know that feeling… 
:eek:
Oh thats great! I pinned mine cause it closed the gap a bit…
No reason not to pin it, I mean you get all the tools and equipment necessary in a nice little package with instructions in English even!