To the OP: There is a lot of static in this thread.
Whether to carry a pony bottle or not is a very controversial topic, as you have no doubt learned (if you have learned nothing else from this thread).
Rather than talk about pony bottle or not, let's talk about the problem you are trying to solve: It's the problem of having a depleted or malfunctioning gas supply at depth. Depleted should never happen -- that's the gas management stuff that people are linking to, and that you should absolutely read. If you consume enough gas underwater to end up in difficulties, you have kind of earned your problems -- you either haven't checked your gas often enough, or you had no plan as to how your gas was to be used.
If you have a malfunctioning gas supply at depth, you have two options. One is to obtain gas from a teammate, and the other is to self-rescue. Self-rescue can be done with a pony bottle, or with double tanks. Virtually everybody who enters an environment where a malfunction could be lethal (wrecks and caves) uses doubles, because they permit self-rescue. If you are frequently diving with previously unknown buddies, or in unknown circumstances, doubles are a great idea, but a pony bottle gives you much the same resource.
There are issues with ponies. If you mount them behind you, you can have a leak you don't realize, and the pony can be empty when you need it. If it's mounted behind you, you can't turn the valve; discovering it's off when you need gas would be depressing.
Another option is to utilize your buddy. There are issues with buddies. Some don't maintain adequate gas reserves to get two people to the surface, and others can't be found, or their attention can't be gained.
I dive in a system that trains buddies. The people I dive with get frankly nervous if they aren't close enough to their teammates, or if they aren't in visual contact with them. In addition, we are taught gas management, and to maintain adequate reserves. Therefore, I do not use a pony, or dive doubles most of the time in recreational dives. But were I to get on a boat in Hawaii with a bunch of strangers, I would sling a 40 as a pony -- the advantage of slinging a bottle being that you can see the valve and the reg and be aware of any leaks, and easily turn on the gas.
There are a lot of problems in diving you have to solve. Precisely stating the problem helps define its parameters, and then you have to decide what resources you have to solve the problem, and choose a strategy. Your instructor has clearly gone one way; you appear to want to go another. I think you have an instructor mismatch.
Even "do you like to set up your own gear" turned into a heated debate.
