I don't buy it. Where's your proof? (Proof that the BB shell on an OEM frame isn't faced to close enough tolerances for adequate bearing life)
There are tens of thousands of production bikes out there with external bottom brackets mounted from the factory.
There is difference between OE installation on a new frame (and new BB shell) and installation on a "experienced" frame that could have been subjected to a myriad of conditions, that could have damaged the shell facing or threads over time. Since this is a mechanics forum and not an assembly factory forum, I will assume the later to be the case.
My proof is the internal memo I got from the Campagnolo Service Center in Italy telling us to remind distributors and retailers to face their BB shells.
My proof is in the 20+ years of wrenching for a living that has demonstrated to me, over and over again, the prudence of chasing and facing.
You may get away with not doing it but, then again, you may not. You'll never know just by looking at the shell (unless the threads are obviously damaged).
As to a new frame: Frames are built without paint (obviously) and the shell may be just fine. Slap some paint on there and suddenly you have a heap of paint that could cause the cups to not sit properly against the shell face. Again, I have seen this enough times to accept it as a fact. I'm sure there are other wrenches on BF who have seen the same thing.