I'm pretty risk averse and conservative as a diver. I don't carry a pony bottle, and here is why.
To begin with, I have the tools to calculate whether the gas supply I am carrying is sufficient for the dive I intend to do. This is the "rock bottom" calculation people are talking about above. To get more information, read NW Grateful Diver's article on
Gas Management. It was his lecture on this topic that changed my diving life forever. As a result, I simply don't descend with a gas supply that isn't generous for what we have planned. An example was last night . . . we met at a dive site where I THOUGHT the plan was to paddle around in 20 feet of water, looking for a specific fish. Turned out the plan was a little different; it was proposed that we go to 70 feet to explore a bottle field, and then work our way upslope and finish our dive decoing out in 20 feet of water, photographing lumpsuckers. We had brought HP80s, figuring they would be more than ample gas for what we were going to do, but with the increased depth, the relationship between total gas and rock bottom started looking iffy -- so we swapped out our 80s for a pair of HP100s we had in the truck, and relaxed, knowing we had plenty of gas for the entire dive and the required reserves.
So, since we KNOW we start the dive with more gas than we need, the only way we could end up needing to share gas would be if we had a catastrophic equipment failure. But in that case, each of us is carrying at LEAST enough gas to get both of us to the surface, and often far more than that. We also practice air-sharing on a regular basis, and do air-sharing ascents, so having to do it for real ought to be a "shrug and get on with it" kind of experience. Since catastrophic gas failure is rare -- we've had one such event in 8 years of active diving -- I'm happy that what we have is enough.
Pony bottles are great for people who travel and have to dive with unknown instabuddies, or for people who solo but don't want to use doubles. I really see no need for them otherwise.