First of all, if you're in a big league locker room the phrase you'll actually hear isn't "stuff happens". However, this is a family site (at least until the kids go to sleep and the goats come out), so we're going with "stuff happens".
The A's have stockpiled an impressive amount of depth. In Yoenis Cespedes, Coco Crisp, Chris Young, Josh Reddick, and Seth Smith, they have 4-5 OFers who are worthy of starting: Basically 2 CFers, 2 LFers, and a RFer.
With the addition of Jed Lowrie to the group of Josh Donaldson, Hiro Nakajima, and Scott Sizemore/Jemile Weeks, the A's also have more infielders than they can fit, at any one time, in the 2B, SS, and 3B slots.
The starting pitching depth chart also goes a lot more than 5 deep amongst pitchers who can lay legitimate claim to being worthy of the gig: Brett Anderson, Jarrod Parker, Bartolo Colon, Tommy Milone, A.J. Griffin, Dan Straily, and Travis Blackley. The bullpen is no different, from Balfour, Cook, Doolittle, Blevins, and Blackley to Norberto, Resop, Neshek, Scribner, Werner and every other pitcher who wows in spring training.
All this, you already know. The presumption, generally, is either that the A's are guarding against injuries or that they are maximizing platoons. I think both are true, but I believe that the "stuff happens" principle is more broad than that. Here's what I mean...
When the A's acquired Young we wondered what the master plan was -- will Crisp be the primary CFer or will Young? Is Cespedes ticketed for a certain amount of DHing to keep him fresh? With the Lowrie deal, we wonder whether it's Nakajima or Lowrie who is likely to get a lot of time away from SS and where will that be? Do the A's have reservations about Donaldson? Are they nervous about Sizemore's knee? Is Weeks non grata?
Here's the punchline, i.e., the thesis to this post. I believe the A's have no idea. They aren't playing anything close to their chest; they're just playing the odds in the same way teams have always done in regards to their minor league prospects. If you want a good major league pitcher, then you need to have 2 really good AAA prospects, or 3 really good single-A prospects. If you have 5 promising prospects you might have yourself 2 big league players.
Turns out the big leagues work the same way, and not just because of injuries. Stuff happens. Someone tears up their knee. Someone has an inexplicably and unforeseen disaster of a season. Someone isn't ready after all, or ages overnight, or can't quite hack it at one position but can at another...
I don't think the A's have any clear idea where Lowrie is going to wind up playing the most or whether it's Donaldson or Sizemore who should be a bigger concern, or how much Young will play LF or how many games Coco will get into. Sure they have some rough idea of what the plan might look like in a hypothetical world where the way things look today doesn't change for 200 consecutive days -- but that's not how baseball really works.
How baseball works is that stuff happens. That's why if you have 5 good OFers, you probably have a good OF. And if you have 2 good CFers you probably have CF covered well. And with 4-5 decent infielders in the "4-5-6" spots on the diamond, you probably have a good infield even after stuff happens, whatever it may turn out to be.
The A's have few stars -- pretty much Cespedes, and to some potential extent Brett Anderson. They have a lot of decent players without having a lot of great players. But what they have is a team that will be nearly as good -- if not just as good -- after stuff happens as it was before. if the Angels and Rangers can't say that, then when stuff happens to everyone it will benefit the A's because the standings are graded on a curve.
That to me, as much as anything, is the new "Moneyball": Perhaps you can't afford stars in an "unfair game," but you might be able to stockpile "decent-to-good players" to the point where you can weather the inevitable speed bumps as a way to pass the Corvettes you would otherwise have to be content to chase.