And you also consider the slim evet of getting infected by a root kit. If that happens to a root user, there is nothing else you can do but format the whole partition(s) on which that distro is installed. Rootkits are almost impossible to remove AFAIK.
So, if you don't have root privilege on your user account, a root kit cannot go beyond your home directory. Also check if
leads you into root console without prompting for password. That is another hazard. If your account is compromised, then logging in as a root will be a piece of cake then.
In short, if you have root permissions, your whole system is exposed to malicious activities. But, if any other user account is compromised then it's not too likely to compromise the whole system, just your account.
I am not so good at the security aspect. Maybe others can shed more light on more issues with using root.
Another thing is some sites are wary of users browsing the net as root cos root has unlimited powers and a expert user can do a lot of damage to the host if he intends to.