This is a game that seemed to be more of a promotional stunt done by Bungie to try to keep the spotlight on them more. You take the roll as a ODST (Orbital Drop Shock Trooper) and you are your squad get shot down and are descending onto the planet, when things go wrong and every one of the team members gets split up. Now its up the the Rookie to find his squad mates and what their fates where. You play as the Rookie as you are just becoming a ODST but soon must take on great responsibilities. The biggest hit to this game was Firefight mode.
Which is like Horde mode for Gears of War 2. It became a very hard challenge and offered people a new experience. The campaign was rather short but had some affect on what else happened where master chief was not at. You get to see a city get ravaged and devastated. And with the addition of then new covenant force a Engineer, they gave all the covenant forces overshields including grunts and drones within its radius. The only weakness they have is to charge a plasma pistol shot and hit them with it, in which they overload and explode, though the pieces that fly from it explode on impact so look out.
Now the other big thing to this game was the Silenced SMG and the Pistol was back with a scope on it and was also silent. Another change was that you had to use Night Vision in some areas, and you could see invisible Brute Stalkers extremely easier. If you were stealth on this game it usually did pay off. Bungie made it to wear if you were using a silenced weapon they would not know where the shot had come from so they would go and scout around. Now the AI become much smarter in terms of battling and knew what to do more often then they did in Halo 3. The main thing that came with this game was for a Beta code to Halo: Reach when the beta launched and for the Halo 3 Multiplayer Disk which came with EVERY Halo 3 map with all expansion maps, including 3 new ones on the Mythic Map Pack II, which wasnt released until not to long ago. So having this was a big advantage. So yes maybe this was a good idea in the first place to just be able to test things out.