That is quite an eye-opener, yet not suprising, to learn that expiration dates are more closely tied to profits than safety.
Sorry, don't mean to hijack the thread with such a long post, but I tend to be long-winded...
Actually, the law behind expiration dates is all about safety. Don't forget that putting expiration dates on drugs wasn't the drug industry's idea, it was a requirement of federal law to protect the public by ensuring that the meds they are taking have the potency that they think they are taking. The law is coming from the side of "Is this medication as potent as it says it is?" rather than your question of "How long is it 'good enough' for?". It's sort of like asking if your car is totally unroadworthy after the 3 years/30,000 mile warranty period. In most cases, of course it's still roadworthy, but the manufacturer can't/won't take responsibility for the car beyond a certain point because of conditions beyond its control like climate or road conditions.
This law ensures that the drug companies have done enough testing to stand by the potency and purity (safety) of their products up to the expiration date. The goal is that every single bottle Extra Strength Tylenol at the drug store really is "extra strength" until the expiration. Beyond the expiration date, however, is "proceed at your own risk". The drug companies haven't claimed that their drugs suddenly turn into inert substances or poison beyond the expiration, although some sources do give you that impression. And if that promotes drug sales, well, the drug companies aren't going to complain.
If someone found that their bottle of pills had degraded to 85% potency within the expiration period, the drug company would be in violation of the law because the drug doesn't have the potency on the label, and believe me, mis-labelling is a major no-no to the FDA and can get your product recalled if the problem seems widespread, therefore, they tend to be conservative with the expiration dating (the drug company sets the date, not the gov't or the law). Of course, I'm not denying that a shorter expiration date would tend to promote faster turnover of supplies, but if drug companies set it too short, consumers would either complain (if only one company makes it) or buy brands or alternative meds that have a longer expiration date so there's a balancing act going on there.
Anyway, so for you as a consumer, it's really "proceed at your own risk" beyond the expiration date, not "It's totally useless" beyond the date. It's important to understand what that expiration date legally represents. But how long something is "good enough" is different for every person's bottle of medicine. You can't put a date on it. And unlike the DoD and its Shelf Life Extension Program, consumers don't have the luxury of FDA scientists to test the potency of actual samples from their aging stockpile of meds.