From a practical standpoint, the only thing I have ever seen fail on a BCG is the gas key (key clogged, screws come loose/improperly staked, screws break). If you have enough rounds through the BCG to trust it; but not so many (over 10k) to put some serious wear on it, then I think you would be fine for 99.9% of problems with a spare bolt. However I've seen some freaky failures (popped primer crushed into gas key) that would have been fixed a lot easier with a spare BCG and nobody likes downtime when you are paying to shoot.
The nice thing about a spare bolt is it is small enough to keep with the rifle and takes care of most of the common AR ailments (extractor, ejector, broken bolt, gas rings). Having said that I have a spare BCG and several spare bolts with my cleaning gear, not because I think I will need them but just because I ended up with extra parts here and there and figure it is better to have them handy then have them at home.