I have a few thousand dives and have never considered a Spare Air... just not enough capacity to get me up from the depths I often dive to. A good pony is my best friend (and I don't mean the four-legged kind).
However, if your diving is largely limited to reasonably shallow dives (above 60-70 ft) and you do not approach your NDL, it could save your life if you can't make an emergency ascent from those depths... and even if you can, it will make the ascent much more comfortable. I've made emergency ascents from 80 and 90 fsw when I didn't have a pony and they were not fun.
However, if your diving is largely limited to reasonably shallow dives (above 60-70 ft) and you do not approach your NDL, it could save your life if you can't make an emergency ascent from those depths... and even if you can, it will make the ascent much more comfortable. I've made emergency ascents from 80 and 90 fsw when I didn't have a pony and they were not fun.