In actuality I think you'll find that MMOs do the same exact thing, only loading what you need to see and the people who need to be seen in order to keep fr up and lag down.. So in a sense loading the game like this makes it more like a modern mmo with out the limitations of servers..

I see...I guess the difference is that it'll only be during public events and in the tower when Destiny uses background matchmaking, rather than throughout the whole game.

Actually at least as far as I know, there will be public zones linking everything together. So it sounds like between strikes raids campaign PvP and well everything else you'll be meeting tons of people every hour