As already said, the most important thing is protect yourself of little mistakes of the day to day. I have a customer where mainly all administration routines are automatized. Each customer open his/her session, works and nobody cares about temp files, disk space and etc. However, sometimes, for any hardware or software glitch, anybody has to execute some administrative function like as remove some lock file which was left for some reason. When the most experienced user went to vacation, he passed his cook book to another person. That days. 1994, the home for root in the system V was "/" (root, of course). So, that person got the cook book, opened a root session and entered
rm /usr/printers/ *
, closed the session and went to his normal affairs. A half hour late I was called because the system was hang. That time, installation were done from qic tape. There was no cds, etc. I spent two days to bring the system back. Today the things are faster and more comfortable but, your lost data is lost.
This kind of mistake on root make us miserable, as any user you will just see one or many messages alerting you it can't obey you.
Other issue is familiarity. If you always can do all on your main system without acknowledge because you are root, when you go to another system, you will feel yourself as a "fish out of water" because all you frequently does will be denied to you. All the instructions you give to somebody won't work because they probably don't work normally as root.
To talk about security, is to be redundant! No one user instead of root can do great harm to your system.