Okay, as pointed out by Bruce Levenson few are going to be surprised an extended version will be released. Extra footage has been a hallmark of all the Lord of the Rings home entertainment release. An extended cut of the last Hobbit film is business as usual.
Home viewing is also better suited for extended cut releases.
There is a huge difference between sitting in a movie theater and watching something on Blu-ray. Really, really long motion pictures can be a little difficult to endure, even when you like the film. The producers of the film generally would prefer the running time was a little shorter because that extra half hour adds up. The accumulated extra time eliminates another showing or two per theater. That can run into a huge amount lost ticket sales. Movies do have to make money, you know.
Speaking of money, getting an extra half hour on the home entertainment release is a huge positive. Consumers do love to get something extra and a half-hour of new, unreleased footage is always a nice addition. So yes, there is a little bit of marketing savvy that goes into coming up with an extended version.