Does it have anything to do with the "Forward unto dawn" or are people just uncreative?
I think the cadet that the thing focuses on (Thomas Laskey) who was on the Forward Unto Dawn during Halo 3. So it will probably have him on the ship in the last episode. He then became the captain of the UNSC Infinity.
Thats the problem, #2 was fluff. It was blatantly obvious that they wanted the entire game to show off the new MP (iir, one of the first, if not the first, actually good xbl mp) and to build for #3. It left a sour taste in a lot of gamers mouths at the time and inevitably gave an uncanny foresight into the future of gaming.
What I didn't like about 3 was that they just played it safe with the story. It was just really basic stuff with Covenant trying to activate the ring and you stopping them and the flood.