Justin Upton trade rumors return for another inexplicable offseason run

Something I hear periodically as trade rumors filter through Twitter: Would the St. Louis Cardinals be interested in Justin Upton, if he were available? Well, yes, probably. Then I have to try to figure out why the Arizona Diamondbacks are interested in trading him. Rule of thumb: If the Cardinals would probably be interested in trading for an outfielder despite having Matt Holliday in left and Oscar Taveras in AAA, it's worth wondering whether the outfielder is really available.

Jerry Crasnick is the instigator this time—here's MLB Daily Dish breaking it out—suggesting that their GM is interested in the "mother lode of talent" he might get in such a trade. I know Family Guy hasn't been cool for a decade, but this is the sort of prospect reasoning that leads me back to this scene every time:

Those prospects he'd get in return could turn out to be anybody—they could even turn into Justin Upton, who was once the top prospect in baseball! Upton's never really broken out, but he's going to be 25 next year, which makes him two years younger than Matt Carpenter. He is a prospect, basically.

So yes—if the Diamondbacks come a-calling with Justin Upton in tow, the Cardinals should probably find a way to make room for him. But why would they come a-calling in the first place?