Tire Replacement and Safety: Prudence or Paranoia?

Lately in my websurfing I keep coming across an often-quoted 6-year recommended tire replacement interval, mentioned by - among other, less reputable sources - the British Rubber Manufacturers Association (BRMA), AAA (quoting the BRMA), and the Ford Motor Company. As one of the multitudes of MO subscribers who will be shodding his hoss with new shoes this spring, I found this sort of information (and the crash cases which prompted this recommended tire interval) to be very interesting. Of course, this recommended interval specifically pertains to four-wheeled automobile tires and sounds somewhat absolute (across the boards, 6 years limit on putting a tire into use? period?), but it started the old decrepit squirrel in my head a'running on its rusty wheel...

Motorcycle tires are physically asked to do quite a bit more than car tires, their life spans are vastly shorter than even the cheapest of automobile tires, and a tire failure on a bike could be quite fatal (not just sort-of fatal). Where are the borders and limits to a streetable tire/set of tires?

At the risk of starting another tire thread, I found a discussion of this sort to be of import due to the fact that:

1. MO has a large readership, with hundreds of MOfos visiting the site at any given point of time. MOrons purchase used bikes every day, often without knowing the history and/or age of their tires. Beyond that, all MOrmons will sooner or later be faced with the decision of when to replace their tires, and what with. A considerable amount of bikers purchase used tires, trusting their local rubber dealer's advice ("good as new; just look at that tread").

2. When one reads through the feature articles at MO, it becomes apparent that a major aim of this site is to guide it's readership in all matters of purchasing motorcycles (e.g., pick a comparo) and related gear, as well as offer insights on rider safety (Pete's warning to lane-splitters). What better place to discuss the safety aspects of purchasing/replacing tires?

That said, what are the generally accepted credentials that qualify a tire for replacement, or make it unsuitable to put into service? Can one put a definitive time interval on tire use, regardless of read wear, or is the decision more of an art form than industry standard? Or does the truth lie somewhere in between? Is trying to squeeze out that extra season of tire life (as many do) worth the risk, or is following an industry standard (if such standards should exist) subscribing to paranoia?

MOraelites?