The problem with that is if you're an administrator and you happen to, say, click on a malicious web page, the web page has full administrative rights too. It can go ahead and install anything it wants on your system - viruses, keystroke loggers, adware, etc. etc. etc. Same goes for e-mail attachments and a host of other risks.
Using an administrative account all the time is BAD security practice. Very, very bad. It's like leaving all your doors and windows unlocked because you can't be bothered to fish a key out of your pocket every time you come home.