Good point. Maybe a better question is, when does storage become wasteful?
Wherever your personal risk threshold is.
Annual likelihood of a given HDD failing: ~5%
Fatality rate of underwater welders
over their working lifetime: ~5%
Annual likelihood of a given home having a fire: ~1.2%
Case fatality rate of H1N1 flu: ~1%
Annual likelihood of any two HDDs failing: ~.25%
Annual likelihood of any three HDDs failing: ~0.0001%
Annual likelihood of being struck by lightning: ~0.0001%
So, if I have my data on one HDD, it's as dangerous as a dangerous, scary job.
If I have it on two HDDs, it's safer than getting the "dangerous" flue strain. But if I only have one backup, it should be offsite because housefires happen.
If I have two backup copies in addition to the original, the odds of data loss are similar to the odds that I'll be struck by lightning. (That happens ~300 times a year, by the way.)
Two local backup copies and a (third) Crashplan backup means you're basically ragnarok-proof.
The apocalypse can sometimes happen.
Going from "lightning strike" to "ragnarok-proof" doesn't really merit a ~30% increase in cost of storage, IMO. If I were a bank, I might disagree with myself.