Are you referring to main or guest accounts here? I thought admin privileges allow to install new apps. I'm interested in hearing about that added level of security.
I use a guest account and am in agreement with the guys about it's benefits. I'm a little anal about people using my computer anyway, so I'll just let them do what they need to on the guest account. Seeing what other people have google searched while on my macbook has gotten me into trouble just a few times
To add to the main topic, if the average user isn't smart enough to do a clean install, who's to say they'd even have an external drive, know about back ups or use time machine? I don't have an external hard drive (just backing data up onto my old PC and iDisk)...