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Put simply (as we always used to say) root leaves big footprints.
Pretty much, there's nothing you won't be able to do as root, whether you intend to or not... that includes deleting needed files caught in a typo or filling your disk up so full that your system will not be able to reboot (you can fill the disk up as a normal user, too - but it's smart enough to leave a reserve).
There are lots of reasons to not do this... pretty much the easiest one - it pretty much defeats all security on the box.
That's pretty much it. You can also use other utilites to control your work as root, like sudo. I like sudo, because you have much more control over commands executed by multiple admins, it will ask for a password for each command (helping you to make sure that you want to do something) and you NEVER have to give out the root password. Giving out the root password to multiple people is just bad policy all around.