You are asking good questions, so kudos to you.
The good news - equipment failures at depth are quite uncommon. That does NOT mean they cannot occur. But, if equipment is properly maintained, and good pre-dive procedures (including inspecting the O-ring on a yoke valve cylinder, or on a DIN regulator first stage) are followed, the chances of a catastrophic gas supply failure are very small indeed. Nonetheless, IF they occur, you may not have much time to reach your buddy to get assistance, or reach the surface if that assistance is not readily available. A good article providing actual hard data on the rate of gas loss associated with different equipment failures is available online ((Life Ending Seconds ADVANCED DIVER MAGAZINE By Curt Bowen)
As you probably note from the majority of responses in this thread, a Spare Air is not considered to be an adequate 'redundant' gas supply. It is relatively expensive considering what you get, and not particularly functional - 3cf of gas, at depth, is so minimal as to be trivial. Even the newer, larger Spare Air, with 6 cf of gas offers very little. Better than nothing? Yes. Anywhere near adequate as a redundant gas supply when a gas supply emergency occurs at 110 ft? Not at all. When I am diving a drysuit, I carry a 6 cf pony bottle as the gas supply strictly for inflating my drysuit. It really isn't a lot of gas.
Carrying a pony bottle of reasonable size (19 cf or larger) is a valid approach to providing a redundant gas supply. Preferences on size, and configuration differ. A 19 cf bottle is small enough to be attached to your primary 'backgas' cylinder, and some people prefer that. I do not like that approach, but that reflects my personal preference as much as anything. I prefer to 'sling' my pony bottle - attached by bolt snaps to a chest strap D-ring and to a waist strap D-ring - because I have better access to the valve, and regulator when it is in front of me. I use 30 or 40 cf bottles, and do not find either to be at all cumbersome or inconvenient, either on the boat, or underwater. The 30 is slightly smaller, and over the years I have gravitated to that size as a (very) slight preference for recreational diving redundancy. For the kind of diving you mention (70-90 feet), either size would work well. I encourage you to make the investment in such a bottle, and a regulator, and the rigging, and learn to use it. After a few dives, you won't know it is there.
And, you should practice with it, to become 'fluid' in switching to the pony bottle regulator. I started learning to carry and use a 40 cf bottle years ago, on a trip to the Florida Keys. (And, yes, I caught some grief from other people on the boat, including the captain) when I carried a 40 cf pony on a 25 ft reef dive. But, that didn't bother me - I wanted to become comfortable diving with a 'slung' bottle. And, I did.Several posters have specifically mentioned O-ring failures, and that concern merits comment. I have seen a few O-ring failures on cylinder yoke valves, at depth. Usually, they are manifested as aggressive bubbling and do not represent catastrophic failures. They are inconvenient, because the dive usually needs to be called. One alternative, to avoid that possibility, is to switch to DIN cylinder valves and regulators. In the western hemisphere, that is not the norm. But, I have used DIN equipment since I began diving some time ago - my first regulator was DIN - and have never regretted my choice.
I also second scubadada's recommendation to do a search here on SB. There have been a number of discussions on this issue, including some that were somewhat 'animated', and they are worth reading as well.
