Between my husband and myself, we have over 1500 dives. We've had two occasions where we've had to share gas . . . both were freeflows. In both cases, it was trivial to establish an air-share and abort the dive. (If we dove in colder water, I would not depend on a buddy's air supply for this, as the dual demand can freeze the BUDDY's first stage, too.) I have two friends in Southern California who have about 5000 dives between them, and have never had to share gas.
I therefore feel quite comfortable on shallow dives, depending on my buddy for my spare gas. It's highly unlikely I will need it, and my buddy is right there if I do. But I rarely dive with strangers, and when I do, I either keep the dive very shallow, or I dive doubles. If I didn't have six sets of them, I'd consider a slung bottle in such circumstances. 19 cf is adequate for dives to 100 feet.
It sounds as though you have a regular dive buddy, and if so, I wouldn't worry about carrying a pony bottle.
