So AT&T is basically admitting their approach to security is "security through obscurity"? As long as you're not a high profile celebrity you should be ok because not one wants to own you...
The main issue is relying on false obscurity, both in systems (your program rot-13s your password) and in passwords (you pick an easy to guess password).
There's no real security failing if you rely on obscurity that isn't exactly a password, so long as you can accurately assess the real obscurity, e.g. port knocking. If, let's say (and this is probably false) AT&T has a billing system where sending 100 specific, not-easily-guessable bytes allows you to get private data, that's no worse than a password, even if the reason that it works is a bug - unless the source code is available to the attacker.
Of course, AT&T's problem here isn't obscurity, it's that they don't want to invest enough for real security at all. Which could be reasonable from a business perspective.