It's used as an in-setting reference because it was introduced as the new year system after the New Republic took over.
So it is actually the dating system used.
So further proof of it being "in-universe" and non-sensical. Why would the New Republic establish a new dating system just resynchronized decades earlier? That would do nothing but throw current economics into disarray and cause chaos. It's not sensible for the New republic to do such a thing, entirely sensible for Palpatine to do so however.
And the argument still stands, why the Battle of Yavin, it does not hold that kind of galactic relevance. Why is it more important than any other battle that has taken place or will take place? This is sloppy ret-conning by Star Wars publishers or the "keepers of the canon" and just plain silly. Will we reset the ABY and BBY after episode 7 and we see what big events transpire then? Just silly.
Just because you think it's non-sensical, doesn't mean it wasn't something that could happen. Real history is rife with illogical things that cultures and governments have done.
The evil Empire reset the dating system due to it's master's egotism. The Empire didn't last long. The New Republic apparently didn't wish to reset back to the dating system of the Old Republic (that was corrupted and fathered the Empire). What is so illogical about the New Republic leaders sitting around a table and deciding on the date of a famous battle that got the Alliance that formed the New Republic going? Again, even if you think their decision is stupid or illogical, see my first paragraph above.
Eta: Some real examples of calendars with a year 0 based on something possibly even less pivotal then a battle that allowed the formation of the current government:
http://en.wikipedia.org/wiki/Assyrian_calendar A temple was built.
http://en.wikipedia.org/wiki/Hijri_year Religious leaders changed their zip code.
http://en.wikipedia.org/wiki/Zoroastrian_calendar Each time a new king was in charge.
Edited by Sturn