It is true...cigars do not go stale, potato chips go stale. Cigars simply dry out. By drying out, I mean the loss of natural oils. Tobacco is good as long as it still contains its natural oils. In order for tobacco to keep its natural oils, it must be stored at the optimum conditions, which is around 70% humidity and 70 degrees. In order for a cigar to lose its natural oils, it must sit in a less than desirable environment for a significant amount of time.
That said, a maker would not utilize tobacco that has been stored inproperly. Instantly, upon inspecting it, cigar-makers and tobacco growers can tell whether or not the tobacco has been stored properly and is of high quality. If not, it is discarded or shredded and sold for machine made cigars.
Tobacco is stored in bales, in special rooms set at the appropriate conditions. These rooms are separate from the rooms used to store and age cigars. Some factories have bales from 20+ years ago, while most major manufacturers run on a 5-year-old inventory. The quality of this tobacco does not change over time, however, the strength and flavor will change. Bales are inspected and rotated throughout the year to ensure quality.
I have been to the Camacho factory and have seen these bales myself. While I personally can't tell a bale of pre-embargo Cuban tobacco leaves from a bale of Honduran tobacco, I can attest to the storage methods described above. Also, Camacho has been using these tobaccos for some time in their Liberty series, and is currently one of the only factories to have genuine pre-embargo tobacco. There are others that purchase pre-embargo tobaccos, like Gurkha, but don't actually own it.