I don't know exactly what Ron Gardenhire's motivations are for announcing that Nick Punto has the inside track on second base next near. Nor do I know what his motivations for keeping Punto in the lineup every day are (though this post by cmathewson may shed some light on it.) For all we know for sure, he's just trying to light a fire under Casilla, to let the rookie know that he's going to have to have an excellent offseason and to make sure to work hard.
But here's what I'm imagining the manager's reasoning to be:
"Well, so far, Casilla doesn't seem to have it. More at-bats would help him... but Punto needs to work out of this slump, so he needs as many at-bats as Casilla does - maybe even more. And since I think Casilla doesn't have it, Punto's going to be my guy next year - I've gotta get him as many at-bats as I can this year, in the hopes that we can get him back on track for next year."
The mistake here, of course, is that, while it explains Gardenhire's actions up to this point - it's a self-fulfilling prophecy. If Casilla doesn't get more at-bats, he won't develop, meaning that Gardenhire will need Punto to get back on track, meaning that giving Punto at-bats over Casilla makes sense.
You'll note that this works the other way, as well, but which would you rather gamble on: that 50 more plate appearances help a minor-league guy make the leap, or that 50 more appearances help a guy who hasn't improved after 550 already this year?
At any rate, when Gardy sends Casilla to Rochester next spring on the grounds that he doesn't have enough experience, don't say I didn't warn you.