even if you spin the hub you should still be able to limp home, by design. Props normally dont just fall off (not saying it doesnt happen though, ive even heard of it locally as well)... that being said, im more interested in carrying an extra fuel water separator, extra fuel filter, strap wrench, fuses, some extra oil, ect... where there is a fairly good chance of something like contaminated fuel ect.
prop is pretty far down the list, just isnt worth sacrificing the storage room, the personal risk, for the very slim probability of occurrence. To cover all bases, add 5 feet and second engine : )