Just because a diver does not like or use a certain tool does not mean that tool is useless for everyone else. I certainly do not condone the use of spare air, or any other air source, for those not trained in the correct and safe use, but I have a hard time understanding why divers would care if another diver decides to carry a 3.0-6.0 cu ft spare air bottle. As @norwhal said above.
I'm surprised this topic isn't debated more often.