I think a pony makes a great deal of sense, particularly in limited visibility or any other condition that may make it harder to maintain buddy contact. A buddy only works if he/she is there when you need them and the relaity is that despite your best efforts they are oftne not in easy reach when you need them. (And I won't even get into the dual fatality issues associated with recreational buddy teams and the risk that poses.)
I firmly believe in divers equipping themselves to be as self reliant as possible so that the inevitable unintentional buddy separation does not pose a significant risk. The buddy system was adapted from surface swimming and, despite it's long history in the diving community and near universal acceptance, it is still far less than perfect. Personally, I think it's ironic that someone would suggest a pony bottle would be a potential risk as it could present a false sense of security when that is exactly what a belief in the buddy system does much of the time.
For example, how many divers have been lost, disoriented or otherwise had no clue how to get back to the boat or exit point and yet routinely rely on a buddy, another diver or DM to safely lead them back. We have created a culture of dependence where a buddy or DM substitutes for a divers ability to get themselves out of a situiation. More directly, we have eliminated the previous culture of self reliance where no diver would ever think of getting into a situation they would not be comfortable getting themself out of.
That said pony bottle use does require a bit of thought to minimize any risk and maximize benfit. The philosophy you have to adopt is that a pony is only to be used in an OOA emergency and should never be used routinely during the dive. In keeping with this the pony bottle shopuld never be used for gas planning purposes and the diver shoud never reduce the reserve they would have maintained without a pony. You also need to dive well within the limits of a pony andsmall ponies (like 6 cu ft) have very little usefulness for a diver. A 19 cu ft pony makes a lot more sense and a 30 is not too much for deeper 100-130 ft diving.
As a technical diver I like slinging deco bottles, but unlike many technical divers who think that since it works well in very demanding tech situations it must be best for every situation and by implication for a recreational pony, I don't think slinging is alwys the best approach for a rec diver.
Back mounting does not allow you to turn the valve on and off at will, so you need to 1) ensure it is on at the start of the dive and 2) you need to ensure your pony reg does not freeflow. However, a back mounted pony is also essentially invisible during the dive and does not have to be managed during the dive as a slung pony needs to be given it's location in front of you. Back mounting also allows the second stage to be routed in a "normal" manner much like an octo.
If you back mount a pony, I would suggest carrying it's second stage in place of your regular octo and also use it in conjunction with an integrated inflator/octo like the Air 2. This lets you or your buddy use your pony like a regular octo (ie: no additional training issues) and still provides you with an extra means to access your primary tank without a 3rd conventional second stage and it's associated hose. It reduces both clutter and confusion.
Given the reality that 2/3rds to 3/4ths of the time an OOA buddy will mug you for your primary, you will have the option of using either the pony or Air 2 when donating in an OOA emergency. And in light of the air sharing realities, a long hose (5ft to 7 ft) primary is a good idea as well.
Much can be made of the advantage of checking the pressure on a slung pony, but the reality is that the pony pressure should be checked prior to the dive and since it is never to be used for non emergency situations, you have no valid reason or need to check it during the dive. If you are using it, it's because you are making an emergency ascent and are out of other options so it either has enough gas to do the job or it does not and knowing the pressure makes no real difference. And if you use an unbalanced first stage the higher inhalation effort on the last dozen breaths or so is ample warning that the bottle is near empty and that terminating your safety stop and ascending is now in order. In most cases a small button SPG is more than adequate for checking pressure on the surface.
If you really have to know the pony pressure during the dive, you can use a conventional SPG on your back mounted pony and route it around the left side mirroring what you do with your primary SPG on the right side.
In the end however, I would suggest you look at your diving, your buddies and your specific requirements and select the approach that works best for you. It's ok to get advice from internet "experts" but in the end you need to consider all the options and select the approach that gives you the most advantage and the most safety under the conditions in which you dive.
