A couple of years ago I went through a phase where I had convinced myself I wanted a pony bottle for my diving, especially the 5% of the time when I find myself with an insta-buddy. But the more I read here, the more it seemed that the better route for me--and my wife, who is my buddy 95% of the time--is to adopt gas planning principles and the philosophy that each of us is carrying the other's back-up gas. We are not photographers, but letting photography or anything else take precedence over good buddy habits seems like a bad idea.
A pony could be useful for the 5% of the time I dive with an insta-buddy, or for when we are ambushed by a divemaster's decision to take us closer to the limits than what we had planned. Sure, if we could plan 100% of our dives ourselves and dive our plan 100% of the time, we wouldn't need a pony, but that's not always the case when doing a DM-led dive. However, the negative I see in taking along a pony for only some of our dives is that (a) we don't always know in advance which dives will take us close to the limit, so we might possibly end up not taking it along when we should have, and (b) switching gear configurations isn't appealing to me; I want to use the same configuration on every dive to the greatest extent possible.
A pony could be useful for the 5% of the time I dive with an insta-buddy, or for when we are ambushed by a divemaster's decision to take us closer to the limits than what we had planned. Sure, if we could plan 100% of our dives ourselves and dive our plan 100% of the time, we wouldn't need a pony, but that's not always the case when doing a DM-led dive. However, the negative I see in taking along a pony for only some of our dives is that (a) we don't always know in advance which dives will take us close to the limit, so we might possibly end up not taking it along when we should have, and (b) switching gear configurations isn't appealing to me; I want to use the same configuration on every dive to the greatest extent possible.