It has been stated (and I cannot find the post at this time) that even the top end parts makers have some amount of bolts that do not pass the proof-mpi. Pair this with the recent rash of busted bolt images circulating the forums as of late and I will gladly pay the $20 premium that comes with a BCM or Daniel Defense bolt.
And for “new and unproven designs…”, despite that fact that some companies know exactly how do make an AR work, they still test and inspect. Testing is not an extra developmental step, it is a verification that good parts are going into the gun.