I've wondered about this myself. I've done .223 for a while, but only recently got an AR-15 (previous was all bolt action; no crimp).
I'm full length sizing with Lee, and I tried to take apart some of my (uncrimped) rounds using a kinetic (hammer) puller on the carpet to the cement..... wow! Pound, pound, pound pound POUND! to get them to come out. I was IMpressed!
I don't have much experience, maybe only 100 rounds so far, but my uncrimped rounds seem to be doing fine.
The chambering movement should move the bullet farther OUT of the round....but because of the magazine limitation, I'm already seating these at SAAMI spec--which is MUCH DEEPER than I'm used to. For the bolt action, I chose .020 off the lands, far farther out than I'm seating these! I shoot fairly mild loads (for target practice) and if the bullet WERE to go out to the lands, it probably will still be well withinn any pressure limit! I've shot hotter loads with bullets into the lands before with a bolt, no problem.
So I'm not real sure why I would need to crimp these? Now, I get a LOT of usage out of one case, since I use fairly low pressure charges, and as the neck work-hardens, then the pressure on the bullet may lesson, and they may move a bit more and change my theory.