Why 40% of worker ants do absolutely nothing

At an afternoon party recently, a Swiss friend of ours, who is a bit of a science geek, mentioned a study he had heard about, which I found fascinating. A University of Arizona study from a couple of years ago found that as many as 40% of worker ants in a colony do absolutely nothing.
We are used to seeing ant colonies as a veritable whirlwind of activity and efficiency, almost machine-like in its single-mindedness and emphasis on productivity. But, on closer inspection, it turns out that nearly half of the workers actually just stand around doing essentially nothing - maybe a little grooming from time to time, a bit of brood care here and there, but most of the time nothing at all.
It turns out, after a laborious exercise of tagging and identifying individual ants in a colony, that these slacker ants are effectively substitute workers, who only perform useful work when the active workers are taken out, be it by predators or natural disasters (or, in this case, curious scientists). This reserve labour force appears to be an essential part of the colony, and if a colony is forced to relocate for whatever reason, the replacement workers are taken with them as though they are an indispensible part of the complex social system of the colony. Fascinating.