Correct, I've ranted about people leaving their larger pony bottles at home several times. If your redundant air is convenient enough to always have it on you, that's far more important than having a much higher capacity you leave at home. (
more info here)
I do also "SideMount" my pony-bottle, but I'm also a SM diver with a SM setup. If I went back to BM, I'd probably continue to rig my pony the same way, because I'm used to SM, and find it quick and easy to clip/unclip/etc, and it's streamlined.
As a side-note, I go back and forth between either 2x 80cu -vs- 1x 80cu & 1x 19cu. Passing a 19cu up a ladder, and then climbing with one tank is convenient. However, sometimes I just sidemount normally, which means one less piece of equipment on the boat.