It's an optional extra measure of safety. Why do some divers carry a 19 cf pony bottle, extra cutting tools, and an SMB on recreational dives and others don't? Because some prefer the extra measure of safety and others do not. Some people jump off of bridges with a length of cord attached to one ankle, others think it's crazy. It's all about risk/benefit/tolerance. What might work for you might not work for someone else.
If you are carrying a redundant gas supply, why not carry one that will for certain let you do your normal safe ascent? And if you don't think that the normal safe ascent is necessary, why do a safety stop at all?"
Because the Spare Air is compact, easy to travel with, and easy to refill "on the go" and is less expensive to purchase and maintain than a pony bottle and full regulator.
So when you fly with it, you have to empty it and then have it VIPed when you get there? You don't do yearly regulator service on it? That's not cheap.
Why is the spare air less expensive? I mean, it's $200. You can put together a cheap used pony bottle and reg for less than that. Maybe you can get Spare Airs used, I don't know. But not that much of a difference.
No, I don't see this diver saying they're going to disregard the buddy system. In fact their original post clearly states they will be diving with a buddy. For the diver who started this thread, a Spare Air is simply an additional measure of safety that will be used only in recreational diving WITH a buddy.
I'm getting confused here, you seem to be going back and forth. You either feel you need a redundant gas supply or you don't. All I'm saying is if you feel you need a redundant gas supply, get one that will let you do your normal, safe ascent. Also, get one that can't empty in less than six seconds at depth if it accidentally gets purged or free flows.
A Spare Air adds extra safety to a single tank rig, and costs next to nothing to maintain on a yearly basis after the initial purchase.
So no yearly reg service?
Thanks for the opportunity to clear up a few misconceptions including the fact that a safety stop is an optional extra measure of safety and "blowing a safety stop" in the unlikely event of a catastrophic OOA emergency does not greatly increase the chances of getting bent. Nor does ascending at 60 ft per minute which is what was taught for decades and only recently was rather arbitrarily changed to 30 ft per minute.
So again, do you ever do a safety stop? If so, why? I mean, if the additional safety is trivial, then why bother, just get out of the water three minutes earlier.
Again, why go out of your way to bring something that won't let you do a normal, safe ascent in case of catastrophic gas loss, and which can drain accidentally so quickly.