|Heyward helps the Cardinals considerably (USAToday)|
But Walden's just icing on the cake, really, because St. Louis clearly got the best player in this deal. Heyward hasn't developed into the Ken Griffey Jr. clone people thought he'd be during his first season, when he was an NL All-Star and Rookie of the Year runner-up to Buster Posey, but he's still a phenomenal player. A true all-around talent, the two-time Gold Glove recipient has twice stolen 20 bases or more in a season and once hit as many as 27 home runs (in 2012). He also owns a rock-solid .351 OBP for his career, during which time he's been worth 24.5 bWAR and rated as the third-best rightfielder in baseball behind Jose Bautista and Ben Zobrist. Plus, he's only 25, though he will be free agent-eligible after next year.
In return the Braves landed Shelby Miller, who's only one year younger than Heyward but won't be eligible for free agency until after the 2018 season. Miller, a starting pitcher, finished third in the 2013 NL Rookie of the Year award to Jose Fernandez and Yasiel Puig after going 15-9 with a 3.06 ERA and 169 strikeouts in 173 and 2/3 innings. His 3.67 FIP and 1.21 WHIP suggested regression was coming, however, and sure enough Miller was merely a league average starting pitcher last year. His record fell to 10-9, his ERA swelled to 3.74, and his strikeout rate plummeted from 8.8 K/9 in 2013 to a paltry 6.2 K/9 last year. His walk rate increased as well, resulting in a 4.54 FIP that says he was very lucky in 2014. Miller may not completely unravel next year, but he seems doomed for more regression and won't have a very long career if he doesn't improve his command. There's just too much uncertainty about his effectiveness going forward, and right now the signs point to bad.
Atlanta's also getting Tyrell Jenkins, a 22 year-old pitching prospect who has yet to pitch above High-A. It's much too early to tell what kind of impact he'll have if and when he reaches the majors, as he'll likely need a few more years of seasoning before he's ready.
So yeah, clear win for the Cardinals here. They get at least one year of Heyward and two years of Walden for four years of Miller and a complete unknown in Jenkins. A six-win player last year, Heyward could very well end up creating as much or more value in 2015 than Miller does over the next four years, especially if the former finally puts it all together to produce that superstar caliber season we've been waiting for. Walden's already an established big league reliever, while there's a good possibility that Jenkins never throws a pitch in the Show.
This trade makes the Braves considerably worse for 2015, which is not the direction they want to go in after losing 83 games last year. St. Louis, on the other hand, remains a safe bet to win 90-plus games again en route to a third consecutive division title. If you're Atlanta, you have to do better than a league-average starting pitcher/likely project and a raw prospect for an elite outfielder such as Heyward. If you're St. Louis, you deserve a giant pat on the back.