I hadn't realised this thread was still running.
I agree that a pony is for emergency only. It is not part of the air calculation. However, it is useful to calculate how long a pony will last at different depths, at elevated breathing rates.
As an instructor, if teaching ocean diver (OW) on single cylinders I always carried a pony (back mounted). A lot of British instructors do this. It gives me redundancy, and independence. I can concentrate on the student, in the event of an air issue I have enough gas to get me (and the students) to the surface. I am not reliant on the student for rescue.
My kit looks like the students. (Unlike my twinset).
It is very unlikely that the issue will be 'running out of air', however, it is not unlikely that it will be a cold water free flow. (Of which there where a few this weekend).
On the rare occasions I dived single cylinder on dives in the UK I would often take a pony. It ensured I had redundancy. Especially on wreck dives where there is a risk of an overhead environment. (UK diving visibility is variable). Generally single cylinder diving was either for shore dive or small boat diving (RIB's and inflatables).
In the old days a predominantly dived twinsets. (Now I use a CCR). In the early days, when Nitrox wasn't readily available, and site selection might change on the day. We often used to carry 3litre ponies of 50%. This was our flushing gas at the end of the dive on a deco' stop. One 3 litre would generally be good for the weekend, 15 minutes or so of deco (on each dive). It was never part of the decompression calculations, or the air calculations.
Gareth