In my quest for a cross bike (to be used as a year round commuter/touring/clubride bike) I have come across an interesting spec item. The Kona JTS comes with a double chainring crank (39-48), but they spec the bike with a triple 105 rear derailleur. Is there any good reason for this in cyclocross or is this a cross-market trick to get people like me to buy the bike?

It worked to sell me on the bike since all I have to do to turn it into a triple is get a BB and a front derailleur. To switch the poprad would have meant also switching the rear derialleur, which is a bit more pricy.