Just to play Devils advocate for a moment...
Yes, it defeats the redundancy purpose if you use the pony to extend your dive. But why is that all they should be used for? That is pretty dogmatic. Why not use a pony for extra gas?
If I am using my 11z ,d a buddy has an 80 on his back, what is wrong with him supplementing with a 40?
As a general rule using the pony to extend dives creates complacency and is poor form from a dive planning perspective.
That said, I would consider/do use the pony in certain situations take can be dive or site specific.
I sling a 40 so obviously not all of these scenarios work well with smaller bottles
1. I have used a pony to stay shallow under the water after a dive has been turned so that I wouldn't have to surface swim. Mostly in situations where the site is benign and it looks like my tanks might end around the 200-300 psi so I'll switch to a pony when back gas is 500 so I don't need to rush out of the water/surface. I still have plenty of back gas and gas in the pony.
2. If I have a half full tank and am going to be doing a shallow no current shore dive or work on the platforms at Dutch at 25'. I have brought that pony and treated it like a stage bottle so I wouldn't have to spend money to fill up a tank that is still half full.
3. Similar to point #1. A buddy of mine wanted to make a deep push into the back of Dutch Springs. I had a HP100. He had an LP108 and a slightly better SAC rate. We planned that when I got low I would switch to the pony. This was a little bit different than #1 bc it was treated more like a stage bottle and I was breathing it for longer at deeper depths.
In my mind, general rules if you are going to use the pony to extend the dive
1. Benign dive conditions--lakes, quarries, shallow shore dives, areas where you can safely surface and swim to the exit. Would never plan on using it if you are doing a wreck in the ocean to many unexpected things can happen
2. Only needs to be deployed after you have turned the dive-It is not the "hey let me drain my tanks and then deploy my pony and decide to swim back" like a J valve. It is in situations where you could surface if you needed to and/or you could finish the dive, but it would mean the tank were a very low on PSI and the pony will be used to conserve back gas PSI and allow you to enjoy the end of the dive and not rush.
3. You have buddies-Using the pony could be a planned part of the dive or just a precaution, but either way you have other divers as a redundancy.
I wouldn't expect everyone to agree with the above and the examples given are really for being used with a bigger bottle--I recommend at least a AL 30,preferably a AL 40.
Some people might consider the above poor gas planning, but I don't think I completely agree with that. There is no deco in these dives and there is no hard set game plan. You say you are going to go see X or try to find Y, depth with be around Z, but its never 100% exact. Only the final destination is set, enjoy the journey.