Consider this for a moment...
If you are looking in the mirror, you are not looking where you are going. You might say that you can do both, but the fact is that you have to focus your attention for at least a short time to actually see what is behind you and process the information. To do that enough to make looking back increase your safety IMHO you are negating it by not paying enough attention to what is going on in front of you.
A car is a different animal. Bigger mirrors that are adjusted and positioned to give easy access to the visual information. Also, you only need to actually look at them when changing lanes, passing, etc.
So... back to the original conclusion - mirrors are of little to no use on a bicycle. They certainly aren't a safety device, and for the novice - the usual users - they are most likely the opposite.