I'll weigh in here.
I dive with a 13 cu ft pony tank, back mounted, on every dive. I started using one after I got involved in underwater photography, and subsequently began diving solo most of the time.
Diving solo, UW photography, and diving with a pony tank are all intertwined for me. But first, the basics.
If one has an out of air emergency, it seems to me that the simplest solution is the best and safest one. If you are out of air or dangerously low on air, the air you need is UP. The shortest distance to UP is UP. However, if you have an OOA situation with a buddy, let's count the steps. 1. Locate buddy (and if you say you always know exactly where your buddy is, you probably don't have a lot of dives logged). 2. Get to buddy. Usually that means going some direction besides UP. Sometimes, even DOWN!. 3. Establish shared breathing of some sort. Hopefully it's just grabbing his octopus and saying a short prayer that he's maintained it, that the purge button works, that it hasn't been dragged through the sand, etc. While you are praying, pray that he has a standard octo rig rather than a octo/BC inflator combo, because that requires step 3.1 asking for your buddy's primary regulator and waiting for that to happen. That means than in an emergency situation, with stress levels high, there are a few moments when both the OOA diver and the donor diver don't have regulators in their mouths. I can't see that as being a good thing under those circumstances. 4. Do an emergency ascent while attached to another diver, where both divers have to find proper buoyancy, and both divers have to agree on the ascent rate (sometimes the dive who ran out of air wants to go up a little faster than the donor diver). Again, this is another time to pray your donor diver uses a standard octopus, trying to maintain proper buoyancy when the fill and exhaust control of your BC is IN YOUR MOUTH takes some practice. Combo users, when is the last time you practiced an emergency ascent while donating air to another diver through your primary?
Now, let's count the steps for an OOA emergency with a pony tank. 1. Put the regulator that is bungied under my chin in my mouth. 2. Go UP.
9 out of 10 rocket scientists probably agree that as far as getting your ass out of trouble in a real OOA emergency, a pony tank has it's advantages.
Here are a few other points to consider: If you are an UW photographer or videographer, you are diving solo. If you are buddied with an UW photographer or videographer, you are diving solo. Under those circumstances, you better have a plan for getting to the surface in an OOA situation that doesn't involve finding the diver who has lagged behind to take some shots and is now further from you than the surface. Just food for thought.
All that said, let me follow by saying that overall, I do believe that diving with a known, reliable buddy, in a situation where nobody is gathering digital images, is safer than diving solo. In my case, I don't know any divers who can travel on my schedule, and even if I did, I'm still taking pictures and as such, I'm oftentimes not going to be close enough to a buddy to count on him. So I had to decide what was the next best alternative, and for me it was equipping and preparing myself to be self reliant under water. I always dive with a pony tank. I carry a knife, and shears, dive alert, Storm whistle, and a safety flag on every dive. I have a Solo diver Certification. I also carry a Personal Locator Beacon in a waterproof canister on any dive where I'm going to be isolated, in current, or away from land. I can activate this PLB any place on Earth and a satellite signal with my exact GPS location is transmitted to rescue facilities within 45 seconds.
So, why solo dive when you have any other option? I can only say, it's different than buddy diving. Very different. Some may like it, some may not, but I do. I really, really enjoy the freedom of diving, and being completely by yourself underwater is like nothing else. I particularly enjoy solo night dives, the serenity is unsurpassed.
I know how volatile this subject is, and I know how passionate people are about it. You can save your breath telling me how your way is better. It may well be, but I'm coming up on about 1000 solo dives, and I doubt you'll be able to save me from myself at this point.
And I can't end without mentioning that carrying a pony tank to remote locations is a flat out pain in the ass, as is dealing with it on a rolling boat and hauling it from place to place and getting it filled and VIPed and Hydro'ed, etc. I just think of it as a basic piece of my dive kit, and if I go, it goes. Then I don't worry about it any more.