If all you need is more gas for contingency, you might as well take a larger tank and leave the pony at home. The point of the pony is that it is an independent source of gas. When does that make a difference?
For example a cold water backmount diver who isn't able to shut his valves due to a rotator cuff injury. Or maybe he's able to but only slowly in a training situation, and he doesn't feel like he wants to rely on that or his buddy in a real stressful situation. As said above, it's also a personal subjective matter of what kind of redundant gas you feel safe with or not.
Many agencies recommend dual tank valves and two first stages in cold water, although most sport divers are unable to close their own mono tank h-valve without help of their buddy. It's a permanent discussion point in German/Austrian dive forums: why do the clubs recommend h-valve mono tanks while so many members cannot close their own mono tank valves. Should we rather follow BSAC and recommend pony tanks instead of h-valve mono for cold water? Note that all these agencies discourage solo diving. Solo divers usually prefer a truely independent redundant source of gas, and many dive with pony tanks, independent doubles, or sidemount.