Why should I use a mirror?
You should use a rearview mirror because:


It allows you to look behind you by moving just your eyes. This will help you not accidentally swerving into the traffic coming up behind you you're trying to get a look at.
You can (almost) look behind you and in front of you at the same time because it just takes a small eye movement rather than a huge head movement to go from looking ahead to looking behind and back.
It actually shows you a useful part of the road behind you despite the small size (try it)
All the other reasons I don't know about for which they are mandatory on motorized vehicles in (almost?) all countries all over the world.
They actually cost less than a good pair of socks.


You shouldn't use a rearview mirror because:


You'll look like a dork, same as with a helmet.
You're an experienced bicyclist and you know enough to stay out of danger
It'll add extra drag.
It'll add extra weight.
They cost more than a cup of coffee.
They'll screw with the sleek lines of you fixed gear machine.


I bought one after a few days of cycling in new zealand. Narrow roads, wide cattle and log trucks with sleepy drivers, some near misses, and constant warnings to be very afraid of drunk drivers made it an obvious choice. Especially because of the swerving point above, which seems aggravated when riding a bike with 20kg of luggage strapped to it uphill. I've used it on all my cycling holidays since.

Then again, I don't use a rearview mirror in city traffic in my hometown either.