Vertical splitscreen isn't inherently bad though, both Resistance and Warhawk do it well. Warhawk's vertical splitscreen is actually excellent, because they mix the surroundsound to create left and right sound fields rather than just mishmashing it all together like pretty much every other console game does.
The Halo games on the other hand have always had a rather narrow FOV, and giving us just half that FOV of full widescreen when running splitscreen is just absurd. I always turned off widescreen to play splitsceeen Halo 2, prefering to run it pillarboxed like Halo 3 does automatically now. So in that regard Halo 3 splitscreen is an improvement for me, but not nearly the one I was hoping for.
They certainly have a knack for strange choices.
I take it that Resistance and Warhawk give you the same horizontal FOV from the SD mode and extend the vertical FOV? That's something I wouldn't mind, especially now that you mention how much easier it would be to separate the audio channels appropriately.