Mori,
I think the benefit of using the AD/BC system (or any other one, it happens to be the AD/BC system anyhow) is standardization. Anyone in the world can go to any other place in the world and say, "I bought this car in 2004." The language might be different, but you can bet most of the civilized world will understand exactly the year you bought your car.
That has quite an interesting economic benefit. Right now I can tell you some problems are coming straight out of the Imperial and SI measurement systems. You say, "I want four hundred wooden beams, each five feet long, shipped from your plant in Buenos Aires to my distribution facility in Kansas City." There is a chance you'll end up with a botched order because Juan Doe didn't convert the numbers right.
I think that the prevalence of the AD/BC system instead of, say, AUC, or the Julian Calendar or even the system the Khmer Rouge used in Cambodia, is really a product of a western cultural hegemony.