Pony bottle size

[Tol]

I'm well aware that I'm tapping into a much bigger issue with this question.

Why not consider doubles? 
The advantages are plentiful, the disadvantages few.  I understand that there is a financial hit, but pony bottles aren't free either.  Instead of slinging a pony bottle as a stage with no inherent redundancy for your back gas, just start your equipment configuration correctly and dive doubles. 

It seems a shame to put more money into kit that will never give you what you need to dive safely.

JMHO,
T