Not wanting to put words in Glass's mouth.
Many say that it is more secure as any malware that gets into your system doesn't inherit your administrator privileges and they can't create registry entries and place files in the system folders and for the most part you don't need administrator privileges.
But, having to switch users when you do need to have administrator privileges plus MS XP doesn't make it a condition that you have a limited user account and only use the administrator account when you actually need it. This can be a right royal pain in the rear so most just log on to their account with administrator privileges. There is however, a way to limit the rights of programs that access the internet without much trouble, check out the DropMyRights link in my signature.