Parsing Joe Strauss rumors is, regardless of their veracity, best left to people with a lot of prior experience on the Soviet beat for Foreign Policy; the man's style has always depended on leaving room for his readers to wonder whether he knows more than he's letting on. But I'm pretty sure the line in his piece that set off such a big trade-rumors comment-storm yesterday--"For the first time the organization might be willing to include elite prospect Shelby Miller in trade talks," emphasis mine--is speculation, and not a coy hint that the Cardinals are shopping Miller.
As speculation, it's fair and probably accurate. The Cardinals should be shopping Shelby Miller, inasmuch as it doesn't cost them anything to do it. The Cardinals have a lot of pitching prospects in the hopper, and a lot of money coming off the books; they could take on an ex-star's contract if they wanted to.
Whether they should make the trade or not (emphasis also mine)--well, I think it's important to figure out what the Cardinals would be trading before they figure out what they'd be trading him for.
Shelby Miller is 21 years old and hasn't blown out his elbow or shoulder yet. That is, itself, exciting news to me. He's having a season that would be a little disappointing--10.5 strikeouts per nine, five walks--if it weren't for the two home runs per nine innings he's also allowed. With those 17 home runs in tow, it's a very disappointing year.
Nevertheless: He's third in the PCL in strikeout rate, behind Trevor Bauer and, of all people, Wil Ledezma, and his velocity is back up, and he's a year removed from 11 strikeouts and three walks and hardly any home runs at all between high-A Palm Beach and the similarly hitter-friendly confines at AA Springfield.
This is still a very good prospect, but I think the specifics are clouding our opinion of him. I also think that in moving him down toward the middle of the various Top 100 lists coming out right about now, prospect mavens have successfully priced in most of the concerns that come with his 6.17 ERA.
So: A Top 50 prospect, say, who's one step from the majors and who could be there as soon as next season. One who has, to this point, not only impressed with his tools or his age relative to his league but with his production. Forget whether the Cardinals are selling low; this is just what they could be selling right now. And personally, I don't see how they do it.
You wouldn't trade a Top 50 prospect because he's having a bad season; you'd trade him because making that trade solves a problem with the team as it's constructed right now. But unless the Cardinals are in on Zack Greinke or Cole Hamels, there isn't a player or a problem big enough to justify trading Shelby Miller.
This team needs another reliable relief pitcher; it needs a mid-grade starting pitcher; it could use a pretty-good second baseman. I'm sure John Mozeliak is shopping Shelby Miller, or at least thinking about it, but I don't know how he could arrange those problems on one end of a scale so that they weigh what Miller does.