That sneaky-ass boss of yours, always sneaking around, creeping up behind you, trying to catch you looking at /b/ when you should be grinding those numbers. Those hard, cold numbers! The spreadsheets, they hold no pity for the working man.
Yah, I'm guessing that if you really need this, your boss isn't going to take kindly to you actually having one. Now STFU and GBTW.
Of course, you could make the argument that there is a safety benefit to having a mirror on your monitor. You could also use the same argument to justify moving your desk so those nosy bastards can't keep sneaking up on you when you are
Note: The mirror comes with a protective film covering the shiny part that has to be removed in order for the mirror to actually function as a mirror. Judging from the reviews of this product on the site, DealExtreme customers aren't bright enough to figure that out on their own. God help their bosses.