Okay lets see where to start. As always I'm going to number this for everyone.
#1. The game is called Halo 3 ODST. I don't know about you but if I find out that I'm playing a Spartan instead of a normal ODST I'm going to be pissed. The game's gimmick is based on the fact that you are not special anymore, you can't jump into combat guns blazing like the Chief (or even Johnson) does.
#2. People are saying that The Rookie's drop supports this theory. that only a Spartan could have survived the fall....what about Mickey. If anything this supports the opposite since he was promptly knocked out after the crash. Mickey however is in action on the bridge with Dutch immediately even after ricocheting off the Rookies pod and being caught in the EMP blast. Is he a Spartan too?
#3. The Rookies information is only classified and simple so we can feel closer to him. Like the Chief we won't see his face (This is based on the fact that we can't take his helmet of in Fire Fight) so Bungie wants us to kind of insert our face underneath that helmet. His name "The Rookie" supports this as well because if we played as Buck for example we wouldn't be playing as your standard ODST, since his been in numerous battles and even says in his Bio that if he ways any better, he'd be a spartan.
Also ODST Helljumpers were supposed to be the enemy competition. Since the ODST's were trained mainly to compete against the spartans in level of combat. I believe it was Anderson? who developed the ODST's because he was jealous that the Doc's Spartans were getting all the funds and attention as the main effective ground force for the UNSC. I read Fall of Reach soo long ago, my memory might be a bit off in the names. Thats how I got my name though..
I think your confusing ODSTs with Spartan 3s. ODSTs were around before the Spartans. The Spartan 3s were trained by Ackerson not Anderson (Think you mixed up the name, there is no Anderson).
[Edited on 07.10.2009 8:16 AM PDT]