One concern I have not seen mentioned is how this would open up a new attack vector for Windows. If Microsoft were to implement a "disable UAC on a per-program basis" type functionality in Microsoft Windows 8, it would mean that somewhere on the computer, the operating system would have to be storing what was allowed to bypass UAC in some kind of trusted application repository database.
Assuming such a database existed, it would rapidly become one of the most-studied entry points for malicious software authors, since finding a vulnerability in it would allow them to code exploits to bypassing your security. Allowing programs to silently run with Administrator or SYSTEM privileges is not a good idea, and would set Windows security back by at least a decade. The flip side to this is if the trusted application repository database was used to whitelist programs, why not also build in the functionality to blacklist them as well? A malware author (or even just a disgruntled syadmin) might decide to use the database to block not just anti-malware software, but things like Windows updates, the base filtering engine, Windows firewall, and oter software that might be critical to securing your computer, or just run your business.
There are already technologies like AppLocker and Software Restriction Policies to control access to programs, and these work quite well, especially when combined with other tools like anti-malware software, EMET, encryption, software firewalls, and user account restrictions, to name a few.
On computers that I administer, I always turn up UAC to its highest level, not because it is a security boundary (it isn't) but because it alerts me when a program needs to perform operations that may affect the computer and its operating system, and I think that's something which is very important to know. Of course, if you want to have a more insecure system, I don't judge, everyone has to make their own assessments of risk and trade-offs in security, but in this case, the idea of having a trusted application repository database on any system I am responsible for is something I would not want to see.