I completely agree that turning it off on any machine not connected to the Web makes perfect sense, and I would too do so in a heart beat. But, since my son sometimes does his homework on my computer and I also maintain a constant web connection, I will leave it on.
This computer happens to run with a major bunch of horsepower, so while I did go through the exercise of determining the savings in milliseconds of disabling all sorts of devices, startups, services, etc, I ended up deciding that the 1.6 millisecond savings wasn't worth being able to use one machine for both recording and general use. I did take the time to write some Windows extensions to allow me to implement 2-click dynamic disabling/enabling of over 70 services to support running as lean as possible - it just hasn't been worth it to me since XP. (wish the MS folks would have written those into the OS).
I do think the way it is implemented is stupid, but then again even without deliberate visitations to porn sites, I have had instances in the past where machines I have had it turn off have picked up viruses, and I would add that even legitimate or seemingly legitimate sites can have lurking malicious code links - not to mention emails from people you know really coming spoofed to look like they are from familiar friends.
So, while I agree in principal on not liking UAC, for any machine hooked up online I would still recommend leaving it turned on - and for the record I too have had a rather lengthy IT career - in about 6 months it will be 40 years of IT.
Perhaps MS engineers can fix the stupid way the thing was built and implemented, to lessen its most of the time annoyance.
Bob Bone