On a single-user computer there's not a whole lot of value in
preventing yourself from being able to destroy other user's
work - which is what you get from the effort of not having
admin privilages all the time.
If you can destroy stuff you can do it by accident, or through a
software bug, or a misunderstanding.
Also, if you really think there are security bugs lurking in the
programs you are running,
The biggest security hole is sitting in front of the computer, and unlike
the others that one is *very* hard to fix. Policy and tools have to
reflect that as the systems get more secure the bad guys will move their
attacks to the next weakest link - the user.
Alan