First off, thanks for looking

I have a few broken shell extractors kicking around in my range bag and tool box. In 12 years and well over 50k rounds I've never had to use one. Am I just lucky, or is a case neck separation a rare occurance in an AR?

Same here, never had to use one... I've owned AR's since the early 80's and couldn't even guess at the number of rounds I've fired.  Some of that ammo has been some pretty nasty surplus, yet I've never had a case separation... But like you, I've got several broken shell extractors within reach, just incase...

I have never seen a case separation in an M16 as well.  And that's after 100,000's of rounds.
However, i still carry a ruptured cartridge case extractor for the 5.56 (available from Brownells, their P/N: 800-741-0015) because I have experienced several dozen with the m60 machine Gun (7.62mm NATO).  And safely removing the separated case neck and shoulder from inside the M60 barrel chamber without scratching or damaging the chamber is difficult without the 7.62mm GI rubtured case extractor.
So I never want to go there with a smaller 5.56 chamber, especially when the barrel is not as easy to remove from the receiver as is the M60's.
By the way, these M60 problems usually involved only the tracer ammo loaded in the belts at 1 to 4 Ball rounds.  The tracers were headstamped for manufacture in 1962, but the ball rounds were stamped 1974.  So this is an indication that brass can give it up with age.  So as our rat-holed ammo ages in their waterproof boxes buried in the back yard, think about some inventory rotation.

I've had case separations in M1A and FAL (both with over-reloaded brass) but never in an AR.  It seems that .223 cases have neck splits before the cases begin to separate.

I've only used one once, had a Black hills case seperate during a match ; luckly someone on the line had an extractor ! But thats one round out of at least 30,000+ .