I don't think there is one. I have looked myself. I think the retailers are just going to try to squeeze multiple purchases out of us. I can't think of any other major motion picture that does not come out with an option to buy the three disc combo.
Disappointed to find out that the blu ray/dvd combo pack is currently not available. However, I saw the combo pack available for pre-order somewhere else, with 3/28/2012 listed as the release date. I haven't read any official announcement, so I don't know how reliable this information is.
I saw something saying that the extended cut of the movie was only going to be available on the blu-ray/dvd combo pack. Maybe they didn't have time to get that ready for the February 11th release or maybe they just want to get people to buy more than one copy. This way people bought the blu-ray only and/or the dvd only and then they will buy the combo pack because of the extended cut of the movie.