What's the much needed purpose?
If I receive an email or visit a website from a trusted source, that webserver could very well be compromised and attempt to deliver a malware payload to my system. From there it can attempt to install its own drivers, change system files and do all sorts of other nasty things. With UAC enabled, each time it tries to do that I get prompted to approve it.
"So and so program is attempting to do X"
WTH? Uh no? And now I immediately know that my system has some how been compromised. With UAC turned off that, a good virus/trojan/malware has the ability to disable your AV software, hide itself, change drivers and network settings, and all sorts of crap that will basically require a system restore to fix.
I will say it again, the very, very, VERY small benefits you will get from disabling UAC are not worth the security risks. Yes you may have a firewall, and antivirus, and spyware checker, and all that other stuff. Good computer security is about a layered defense and UAC is a very good layer to have. Yeah it can be annoying at times. You know what is more annoying? Having to spend hours reinstalling Windows and all your programs and data.
So please, don't disable UAC. If you need write/change access to game files then follow the instructions above and give your users or user account full access to select folders.