samhayman
I think the OP wants to toggle between track A and B - instantly.
I could be wrong but with Excl Solo you would need to un-solo track A and then solo track B which is not as instantaneous as one would like it to be.
EDIT: me thinks I need to look into this Exclusive Solo thingy then :)
Exclusive solo will only solo ONE track at a time (generally). So if you have track A and track B and have exclusive solo turned on, you simply click 'Solo' on track A, and you hear track A. Then you simple click 'Solo' on track B and it will INSTANTLY and SIMULTANEOUSLY turn off the solo from track A and turn on the solo from track B. This results in the PERFECT A/B tool at the click of a button. I'm sure this is one of the big reasons this feature was added. There are other uses, of course.
Suggest you check out Dim Solo too if you are not already aware (if you are not aware of exclusive, it's unlikely you are aware of Dim Solo, as the features are located next to each other). Dim solo will turn down all tracks that are NOT soloed by a pre-determined amount (-6dB, -12dB and -18dB). Great for assessing the quality of a take while in the context of the music, without having to adjust the faders and screw with your balances. Also can be used to maybe make finer compressor (or other tools) adjustments whilst still in the context of the music and many other purposes.
These are great tools and I use them both regularly.