I'm a big fan of A.P Valves products. I learnt to dive in a Buddy jacket; after qualifying, I bought myself a Buddy jacket; when changing to a different setup, I replaced the jacket with a Buddy wing. One of my favourite bits of kit is the Buddy self-inflating dSMB. etc etc
But there are two Buddy BC 'standards' which I'm not a fan of:
They aren't the only manufacturer who puts these options on their jackets, but they're the one you're most likely to run across in the UK.
The main problem with these features is that they're unnecessary. If you have a well thought-out setup, you should never have any need of either of them.
When I bought my jacket, it came with an emergency cylinder, as standard. I thought it was a good idea at the time. I heard a few arguments against them, but didn't really pay them much attention - suffering from the "I've bought it so it must be good" affliction, mainly. After all, it would give me several breaths of air in an emergency, how could that be a bad thing?
After a while, I came to consider buying a wing. I had to decide if I wanted to spend the extra money on getting it with the cylinder. So I had to consider the matter objectively.
In the event of an emergency where I absolutely have to surface very fast, the bottle could indeed be of assistance. You can fill the jacket in a second or two with a bottle, it takes much longer with the direct feed.
But why would I ever want to inflate that fast? Rapid ascents are a risky business. You should be planning for ways to AVOID ascending fast, not trying to find kit to make it easier.
The cylinder can supply air. You breathe air out of the jacket, then replace that air with the cylinder.
But what will that get you? It'll screw up your buoyancy, only give you a few breaths, and be very task-loading. If you're deep enough to be unable to surface without getting a few breaths in on the way up, you should have a redundant source of air with you. The cylinder is false security if you don't have redundancy and unnecessary if you do.
Finally, there's the panic factor. When things go wrong suddenly, you react as you've been trained to. And I have never been trained to use the emergency cylinder. In the event that anything goes wrong, it would never occur to me to use a mini cylinder. Since a mini cylinder is only for use in an emergency, and I wouldn't think to use it in an emergency, it gave me no benefit.
And since I couldn't actually think of any scenario when I would want to use the bottle, I couldn't even train myself to use it.
When I bought my new Buddy wing, I bought it without the bottle. I don't miss it. A pony or twinset is a far more reassuring bit of kit.
I also bought it without an Auto Air. I've never owned one. I've seen them used, used them on occasion, and talked to their users, and come to the conclusion that they're a bad idea.
There are two ways of using them: With an octopus, or as an octopus.
If you already have two demand valves, you have all you need. Adding a third is adding unnecessary complications to your kit. A regulator is far more likely to fail than an inflator unit. The risk of it causing problems is far higher than the risk that you may someday need three regulators at once. Any equipment that adds more risk than it removes is a bad idea.
So that leaves you with the Auto Air being your backup regulator. For various reasons, the only way to practically do this is to hand over your main DV to your buddy whilst you switch to the Auto Air.
You now have the task of ascending to the surface, with your inflate/deflate control in your mouth, with your buddy right next to you, breathing off your main DV. It's possible, but it'll never be easy.
Then there's maintenance. Having a regulator permanently attached to your BCD makes it more prone to damage than a normal regulator, but also ensures it gets the least amount of maintenance. BCDs get serviced a lot less often than regulators.
Lastly, there's that nasty extra button. An inflator has two buttons: Inflate, and deflate. Even if you've never seen the one your buddy is using before, you can tell which is which underwater almost instantly.
But with an Auto Air, you have a purge button as well. Pushing on this will cause bubbles to come out of the exhaust, but will not let any air out of the BCD. Unfortunately, this looks exactly like action of the deflate control.
I have seen instructors who did a buddy check just minutes before push the wrong button whilst demonstrating a Controlled Buoyant Lift and ascend far faster than they intended, purging air like crazy the whole time.
That was bad enough in the swimming pool. I'd hate to see it happen from 30 meters down.