It makes sense that the main story mode's access would be limited. Based on what I've gathered from recent interviews, one of our primary tasks will be stress-testing the loads on the system, which would suggest a focus on multiplayer content such as raids, strikes & PvP. Also, I would imagine that general bug-hunting could be done just as adequately through exploration, leaving Bungie's in-house beta testers (and anyone who's signed a NDA) to test the story-driven content (which is, in itself, a much smaller & more linear content stream than the others).