I'm using a phone so I'll be brief. You know the x code they're on other xbox things aswell. Take the ODST Reach Beta leaflet for example. It has
Part No. X16-04017-01 on it. Sorry if the zeros have dots in them phone sometimes does that.
Yeah, I think we (or at least I) know that already. It's practically on everything that is made by Xbox or Microsoft Game Studios. For example, Alan Wake and Crackdown both bear the same codes on the back of their cases, and when you buy a new Xbox, even the handles and padding have it over them!
It might be that Bungie is a bit overdoing it, or that it is really that important. It may be something in between, though...I mean, who would bother putting it on a barcode? xD
I think it is something like a part number, so it's obligatory.
Because, for example, the Limited Edition has several parts (the journal, the bubble-wrap, the case, the disc, etc.) and is to be assembled by robots , every part must have an ID, somewhere, somehow.
I guess that's just it.