Agreed. At a certain point I would have to imagine it's simpler for inventory and production purposes to just churn out a chip with all frequencies. I compared the three models and it looks like the AT&T version has band 30, as mentioned, but other than that there is no difference. Does that make the AT&T model the "superior" version?
Also, I'm assuming the AT&T model (A1633/A1634) is GSM-only while the other U.S. model (A1688/A1687) has a dual GSM-CDMA chip?
I suspect you're right about having a single model. That said, AT&T alone has been rolling out Band 30, so maybe one of two things happened:
1) Apple had already arranged for a lot of chips without Band 30 already, so is basically putting those in a bunch of phones until supplies are exhausted given the relatively late notice from AT&T
2) There's some additional cost (other than existing inventory) to including Band 30 so they don't want to pay the additional royalty or whatever unless needed.
Bigger question - when will locked phones end? Every carrier is moving away from contracts and instead have the phones separate, with some sort of finance plan. The justifications for locking drop every year.