sounds like cheape insurance to me.
No. In insurance terms it is very expensive poor coverage.
It isn't so much that Spare Air is bad.... it is that it is the worst, in a wide range of possible solutions and options.
This is to me is a very good and reasonable statement.
In the beginning I also considered a Spare Air. I asked my local LDS what they thought and they made two arguements that made me reconsider:
1.) A spare air cost almost as much as a pony/deco bottle but would be limited to shallow recreational diving only. After I progressed past that point it would sit on the shelf never to be used again. A good sized pony/deco bottle could be used throughout my entire diving career.
2.) The LDS would not service the reg set up so I would have to send it out myself for repairs and annual servicing. Same with the hydro.
I know #2 may be conditional for your area/LDS but certainly worth looking into when considering the purchase.
I also though of redundancy in these ways:
a.) With proper gas management I should not go OOA. Was I wanting to use redundancy to mask a skills deficiency?
b.) For a simple problem at a shallow depth I should be able to CESA. Was I wanting to use redundancy to mask a skills deficiency?
c.) For a serious problem at any depth that kept me from CESAing was my redundant volume adequate to resolve the issue?
d.) OOA aside, most equipment failures at depth do not result in an immediate loss of gas to the diver. Was messing with a wee little bit of gas better than simply diving my failing rig to the surface?
e.) When choosing a piece of safety equipment was I going to aim for the least coverage possible or the maximum reasonable amount. What would cause me to go for the least if the prices were comparable?
I now see my redundant air source as an insurance against the rare catastrophic failure at depth that causes me to lose all my back gas quickly. I prefer to have enough volume to be able to ascend calmly and safely so that a simple equipment failure does not result in a medical emergency.