I get what you're saying about Destiny being an MMO, but so far as it seems from Alpha footage, access to Crucible happens during the menu screen, not from the open world. I agree with you, MMO's are not split screen for a very good reason, especially if it would impact the player's experience of the open world (i.e. rubberbanding to where your friend is because you've strayed too far). However, if you don't need to engage the open world to access competitive PvP, then why not enable split screen? The devs made it a separate game type, why not enable unique features?