To answer my own question, it would seem Bungie decided early on to lock the mulitplayer online experience down on a platform basis. Presumably they did this to minimise the overheads required to provide their so-called mesh networking. It is likely they won't touch that because it seems pretty fundamental to their game design BUT that doesn't mean it is an ideal outcome for players. Perhaps you could argue that matchmaking is easier and produces better results when restricted to individual platforms but since it is really just a set of algorithms and user data that seems like a bit of a cop out. The only rationale that really holds water is incorporating universal cross platform play would add a significant development cost that ultimately would produce no additional income. After all Xboxers will buy the game and be quite happy to play against only other Xbox users and same for PS4. The game or content is for most players, secondary to the physical console. If you want platform independence Destiny isn't the game. This is despite all their talk about the social or shared experience being central to the game.

Well how exactly would you recommend integrating them?