My initial, knee-jerk reaction was, "That's a bad idea." And then I sat down and tried to think through WHY I thought it was a bad idea. After all, people with a higher level of training not infrequently dive with a bottom stage to extend dive time, or because the gas in their main supply is not one they want to use on the second dive. So why does the idea proposed in the OP not sit well with me?
Well, I came up with a few things. First off, if you are going to include your pony bottle in your gas planning, you have to think about how you have that pony bottle configured. A lot of people fasten their pony bottle to their primary tank in the back, and don't include a pressure gauge that measures what's in it in their setup. If you are going to include the bottle in your gas plan, you HAVE to have a way to know how much gas is in it. Yes, you think you know what it had when you got into the water, but if the reg has had a slow leak you haven't noticed, you may have much less now. People who use bottom stages sling them so that the valve and regulator are visible and easily reached, and put some kind of pressure measuring device on the regulator so they know what's in the bottle.
But that brings up another issue -- people who use bottom stages, use them FIRST. The gas you have on your back is securely attached to you, whereas gas that you have in a sling tank can be lost. (This is the downside of not clamping the bottle to your primary tank.) So you START on the auxiliary tank, and go to backgas if you need to -- although a lot of the time, the plan is simply to execute the dive on the slung tank, and not touch backgas at all. But it is your reserve, and you know you have it with you. And starting on your auxiliary tank doesn't solve your problem of avoiding fills.
In recreational diving, it's generally not a good idea to create a dive plan that PLANS on someone running low on, or out of gas in a supply and having to switch. Cave divers do this -- they breathe down stages until they are nearly or actually empty, and switch -- but they also have extensive training on how to do switching procedures, how to watch multiple gas supplies, and on coping with having a regulator out of their mouth or dry for a period of time. In addition, they often do gas switches where their teammates can watch them, and everyone on the team is extensively schooled on air-sharing and donating quickly and efficiently . . . because we all know that probably the most frequent cause of an out-of-gas situation for a cave diver is not that they are out of gas, but that they do not have quick enough access to whatever gas supply they do have. (In other words, something in a gas switch got messed up temporarily, and they need somebody else's regulator for a minute or two to get it sorted out.)
At any rate, I came to the conclusion that I really did have enough reasons to say that, for a recreational diver without further training, a gas plan that involves a planned gas switch is probably a bad idea, and that doing it simply to avoid paying for a fill is a bad reason.
and if i was the one that payed for my gas id probably walk to half the places i go. 
