It's common because most countries and cultures can or will use it as a dating system in the modern era, however, it started being called Common Era to be less biased towards Christianity while not making us all get new calendars. Sure, it's an arbitrary designation of dividing history. One might also argue that "AD" is also irrelevant in regard to Christianity, since it's off by 4 years anyway.

But it is more considerate, I suppose, if we are all going to try and use the same calendar system.