You also have to think about the gameplay reveal on PS4 in terms of marketing as well. Most Xbox players are familiar with Bungie and Halo. Their purchase of Destiny is pretty reliable. But the average Playstation player is less likely to be familiar with Bungie's pedigree and may be less likely to buy Destiny. So by spending extra time with the Playstation crowd, Bungie is expanding its player base. I know as hardcore fans, it's hard to think that gamers at large wouldn't be familiar with Bungie, but there are plenty of people out there far more interested in other games and developers. Also, Bungie presenting Destiny at the Playstation conference is a plus for publisher Activision. You know Call of Duty (Activision's other mega-franchise) will be a highlight at the Xbox conference. I would argue that COD has its strongest player base on the Xbox. And to also include Destiny in the Xbox conference may oversaturate the conference with both franchises. Having Destiny be a major reveal for Activision at the Playstation conference lets COD and Destiny both get big attention without directly competing in the same conference. And to the OP's point, Xbox fans can expect the same game on Xbox One. Plus, regardless we are all getting a gameplay reveal!