This game is going to be set up like an MMO when it comes to introducing new content. Every few months, new content will be added through patches/dlc/expansions, which will give the game a steady flow of new things to do so we aren't spending too long grinding the same raid over and over. So if you ONLY want to play the very tip of the game, don't bother with the expansion. If you want to actually play a gear progression game as intended, then yes. It should be picked up within a couple days of release