There's a cynical way to read the St. Louis Cardinals' decision to demote Michael Wacha, and not Tyler Lyons, ahead of Jake Westbrook's return to the rotation, and an earnest way, and both of them involve the Cardinals being a better baseball team later, instead of right this instant. (Here's Rick Hummel with the news.)
Cynical: Service time, service time, service time. For whatever reason, baseball has chosen to use the power of economic incentives to delay the introduction of its most exciting rookies as long as is possible. The Cardinals bucked the system by calling him up at all; now that he's been perfectly adequate, instead of as good as all the Cardinals' other starters, they have no particular reason to leave him at the back of the rotation, accumulating service time.
I don't like this explanation, which is not to say it isn't accurate.
Idealistic: Now Michael Wacha can Work on Things! I'm already on the record as saying Wacha looks a lot like a finished product, but he's also a 21-year-old who's made nine minor league starts in his only real professional season.
Tyler Lyons also has things to work on—probably more of them—but it's less important to the Cardinals' future that he has a chance to do so.
What's interesting about these two very pat answers to the Michael Wacha question is that neither one of them is about ensuring the Cardinals are the best team they could possibly be right now.
And that's the real answer: Right now, the Cardinals don't need Michael Wacha to [GET EXPENSIVE FASTER]/[RISK STUNTED DEVELOPMENT] at the back of the major league rotation. The Cardinals are far from safe at the top of the NL Central, but they've got 3.5 games of slack between them and the Reds. However determined you are to break baseball-economics kayfabe, this looks like them using it.