By the power of truth, I, while living, have conquered the universe.
Posted by: SavXtreme
If you want my theory, I say Bungie are doing a bit of trial and error with Halo ODST.
They've tried and tested using spartans, which obviously works extremely well. However now they're experimenting with using a more vulnerable and weaker character such as an ODST (main difference being the lack of jump height, speed and shields).
Of course we all know some of the key factors to Halo gameplay is jumping, movement speed/fluidity and shields. With the contrast of ODST and Halo 3, Bungie can actually see (few months down the line after ODST obviously, from seeing feedback), if the ODST character varient works better than a spartan.
I say they've spotted the success of other shooters such as CoD which use no shields and low jumping, and seeing if it can provide a better gameplay experience for Halo. It's worth a try and it also gives us a new experience in the form of a new expansion for Halo 3.
Who knows, I'm sure they've planned Reach to be played as spartans, but perhaps if Halo ODST works exceptionally well then they may switch the characters to ODSTs instead.
Reach has been in development for abut 2 years now, too late to change the story probably.