Spare air is not suitable for the situations which you describe. Twin tanks, irrespective of capacity, provide redundancy for potentially hazardous failures and general foul ups. Some alternate equipments for relatively deep, overhead environments are H valves and pony tanks. A pony tank is just a large Spare Air.
The spare air should be useful for fairly deep dives in open water provided the diver is not entering decompression and does not consider "safety stops" as essential. For younger divers in good shape the safety stop and 30 ft/min are irrelevant. So, yes, the Spare Air can be useful for reef dives and soforth down to about 100 feet. Free divers regularly dive to 60-100 feet BOTH WAYS on one breath, so the average, competent diver should be able to ascend from 100 fsw with only a Spare Air without a problem. Further good news is that it is unlikely that you will ever have to depend on the redundant air supply unless you screw up badly.
In fact, it is quite possible to ascend from that depth without any redundancy if the problem is merely an empty tank. Actually, the tank is not empty, it just breaths harder. Using a Thermo valve with its small 0.100 drilled orifice would likely ensure enough air for the ascent. The Sherwood valve has a larger orifice and might make this problematic, eg, the wakeup call might be a tad late. If the problem is a blocked valve or blown disc then a Spare Air could save your life.
There is a moral hazard issue with redundant equipment. As with automobiles, diving gear which is rigged to "save the day" seems to encourage divers to look for trouble when they should be exercising caution. The believe that more gear will save them even if they are short on brains.
