If you were to randomly look at 10 residential vanities, how many do you think would have enough counter space to install an appliance garage large enough to hold a razor, dryer, curler, and possibly one or two other items. Usually when you come across a large vanity it has two sinks, which negates any possibility of big open areas. Also, many mirrors would tend to interfere with an appliance garage that sat up higher that the back splash. Large mirrors above the vanity are the reason that most electricians avoid even installing a receptacle directly above the back of the vanity.
Don't get me wrong; an appliance garage would definately "work" in some cases. It just won't be as universally acceptable (visually and practically) as it seems to be in kitches.
