That's a good point about fire forming improved brass; I hadn't thought about that, and I even have a 30-06 Ackley Improved rifle.
As to cracks, even if it was it's not really a safety issue. Having fired lots of surplus ammo over the years, even stuff from the 30's and 40's, I've had plenty of split brass. Like I mentioned before, it's damage to the base that you have to worry about, but split necks and shoulders are not typically serious because the body will still seal the chamber.
Not that you should ever intentionally fire a cracked case, that's a bad practice. It's always best avoided, but on rare occasion I've even had new, factory fresh brass split on the first firing.
I honestly don't know if a split case does any damage/erosion to a chamber. If so I'd assume it's minute. Most of the rifles I've burned surplus ammo through have been old Mausers and Mosins and the like, and I've never noticed any harm, but I do try to avoid it.
I'm agreeing with you there.