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