Maybe from a different point of view may help:
I like knowing when something wants to access my system files. That's how most stuff gets in, by overwriting the system files.
If I disable UAC, how will I know if that new program I've downloaded is trying to access files it shouldn't?
BTW - with most new installers on legit software, they offer the choice between being accesible to all users or being accessible to just the current user.
If you choose to install for just the current user, many installers won't bother you with UAC.
As an added bonus, the installed software gives you a UAC prompt if malware hooks onto it, and then you can know if something is wrong.
If you're on a single user system, it's a good idea to install most stuff for just the current user. Less UAC prompts.
UAC isn't bad, especially on Windows 7, and especially with newer installers.