Not sure why you would need zero offset arms any more than offset arms. Sure, that would be fine as it would then work with offset C-bushings. Just like putting them in backwards to reduce the arm offset.
So nothing wrong with building the caster correction into them either, if that's what would work with a particular Bronco. Put the C-bushings in whichever direction works best.
As said, if it's too much, or if you're going to cut-n-turn anyway, then just put the bushings in backwards to correct pinion angle, turn the yokes to get the correct caster and you're done. Same as zero offset arms.
No need for them to make different part numbers with different caster offsets then, just to get the same end result via a different method.
Unless you prefer to run rubber C-bushings that is. But that's rarely a good idea for anything but a casual use street driven Bronco anyway. Poly is the way to go for harder, or questionable uses. With no price penalty for different offset ratings.
So neither here nor there as far as pinion angle is concerned.
Maybe there are other reasons? But rubber bushings is the only one I can think of off hand.
Speaking of which... Someone should make rubber c-bushings with side flanges to keep them retained in the arm caps. Maybe a little metal reinforcement for the shoulder. Best of both worlds possibly.
Paul