If you maintain your gear properly, including inspecting hoses, watch your spg and don't dive in a way that you're too deep to make an ascent with the amount of remaining air you have, odds are that you will never have any of these issues.
Because there is not a realistic scenario possible were I will need one.
As both of you guys are based in areas where the water is cool, not cold, I'll cut you some slack....
In these parts, a catastrophic reg freeze-up is a distinct possibility for about 1/2 of the dive season, and a reasonable possibility for the rest of it. And before you dismiss this with a comment about "proper cold water regs" etc, bear in mind that I see this most often in relatively newer divers. Most of us that dive in Tobermory regularly and for an "extended season" wear doubles, or dive SM. It's not because we need the extra gas, it's because we want the redundancy of a completely separate reg and gas supply.
A pony is a simple "entry level" redundant system. When I was actively teaching, I encouraged students to include a pony and cheap, cold water reg as a "required" bit of kit.
To give you a specific example, last spring I was at one of our wreck sites, mid May. The wreck lies in 105' of water, but most of the dive would typically be conducted in 90' or so. Very much an advanced recreational dive. Water temp at the surface at the time of year is typically about 40°/4° and about the same at the bottom, although it can be as low as 37°/3° and it will never be above 42°/5° all year, other then when there's some weird current blowing through.
The morning we were there, there was a charter with I would say 8 divers on it. I didn't know them, but they were all in dry suits, so presumably "advanced" divers. Every single one of them froze up within 10 minutes of heading down. Thankfully, many freeze-ups happen on descent, when a diver makes the mistake of laying on their inflator and inhales at the same time. In those cases, typically, it's pretty simple to beat a hasty retreat to the surface. If you're watching from another boat, you can see the geyser of bubbles long before the diver rockets out of the middle of them. The odd person is completely freaked by the experience. Most are shaken. A few laugh it off, and once in a while, someone embolizes and dies. There was a 31 year old woman that embolized this spring after a freeze-up in 100'. She's been in a vegetative state ever since.
If the freeze-up occurs later in the dive, when the diver is say at 1000 psi and heading back to the upline, he's got a real problem, because that 1000 psi will blow through his regulator very quickly. Plus, he now has the added worry of a potential decompression obligation or close to it. Guaranteed their ascent rate is going to exceed 30 fpm and a safety stop isn't happening. Best case is that they have a near miss. Next best is a bend, and last, is they run out of gas in 60', panic and die. To quote our local Hyperbaric Physician, "We can fix bent. We can't fix dead."
Incidentally, when I worked in Tobermory, and before true cold water regs were really a thing (Other than the Blizzard) double fatalities were common and outnumbered singles. That was because of the proverbial "cascading events"... a freeze-up led to air-sharing which led to double freeze-ups and double embolisms. We lost 12 people in two years here. That was a long time ago, but my recollection is that ever one of them involved a freeze-up and most began with one.
Sorry for the long reply, but a pony and a second reg has the potential to end a serious event. It's not perfect, but it's a great help.
And no matter how well someone maintains their gear, a freeze-up is always possible. I had one at 170' a few years ago, and even with full redundancy in my doubles, AND a slung bottle of 50%, it's a puckering moment. It's great you can shut down one post, but if one reg freezes, the other one likely isn't too far behind, but it sure beats nothing!