It would depend on what you have installed - for example, some security minded software need elevated privileges to work correctly. Secunia PSI is a notorious one - and they haven't figured out how to make it run in the system tray without asking for elevated privileges, but if you let it load with windows then Defender blocks it from starting up because it requires elevated privileges whenever it runs. That is *Secunia's* fault, not Vistas. They have had 3+ years to develop apps which are UAC compatible - and haven't. Furthermore, being from Secunia, you'd think they'd already know better, as the ability to have system wide privilege levels in XP is what made it so easy to exploit - and Secunia is an exploitation research firm....
If this method of doing things were troublesome, or flawed, then business would not have been doing the same thing since NT 3 - running users with limited privileges. UNIX would not have done it. Linux would not be doing it. Netware would not have done it.
UAC is good - deal with the prompts, or else replace your software, or else be prepared to have a *lot* more of your system resources eaten up as you struggle to keep your system intact using HIPS, additional FWs, anti-malware (which I include anti-rootkit, antivirus, anti-spyware, etc in) and more.
Mind you, this is not meant to be derogatory or aimed specifically at *you* Carl, but is a wake up note to every one in the world.
IE 6 had the ability to automatically allow ActiveX installations without the prompt - same for cookie storage, file downloading, etc.
All of a sudden, there was a rash of drive by infections, hijacked web pages serving malware and trojans and such automatically to victims' computers without them knowing about it, etc. Even back then I always ran my IE so that it alerted me to every action it was going to take - after all, *I* am the human being using the computer - the computer isn't *smarter* than me - it may be faster at performing calculations, but unless I program it to calculate 2 dimensional matrices, it cannot do that - and it cannot learn how to on its own. I can - and have - and thus, *I* control the computer. I don't let it take many things for granted - I allow what gets started up automatically, I decided which cookies I keep, I decide which sites are allowed to use JavaScript when displaying content.
It's called responsible computing. Just as with your car, you are responsible for its maintenance, with your computer you are responsible just the same.
I did get quite a fair few virus`s and malware while using XP and being able to block its installation is a huge improvement considering how many e-mails I get loaded with embedded code trying to exploit some crap 
