Bob, the reason is that I shoot for publication, and the commercial print and advertizing media deals with a different color space than those optimized by computer screens and the types of printers the average person buys. Computer screens and most printers are optimized for sRGB, while commercial printers are optimized for aRGB.
I've set my camera to shoot in aRGB and PS to process in aRGB so that I can submit images that haven't been converted, and so that they are more pure and don't even have to undergo any conversion process by the publisher.
I've also calibrated my monitor so that I get true aRGB colors.
The only reason I might convert my aRGB images to sRGB, or "dumb them down," as you put it, is for posting them on the internet or for printing them on my cheap, little printer.
The printing and advertising industry has always operated this way, and instead of using a simple (s: standard) Red/Green/Blue color space, they use a "four-color process." Even back in the days of Kodachrome slides, they had to be converted to this four-color process, producting chromalons that required further color correcting.
Basically, aRGB has a broader gamut, more color gradients and creates a much larger file with more color information.
sRGB is the standard and the right choice for at least 90 percent of photographers. Unless you plan to shoot specifically for publication or for a major stock photo agency, there's no reason to use aRGB and plenty of reasons not to...
To tell you the truth, I'm not sure I should even use it. That's what I'm still trying to determine.