In 2008, Ubaldo Jimenez threw 198.2 innings with 4.67 walks per nine. He managed 3.8 WAR. The next year, he cut the BB/9 to 3.51, and upped his WAR to 5.3.
In 2009, Chad Gaudin threw 147.1 innings with 4.64 walks per nine. He managed 1.8 WAR. The next year, he cut the BB/9 to 3.44, but his WAR fell to -1.1.
Walks are fickle mistresses. In the last decade, 27 pitchers have pitched at least 100 innings with at least 4.5 BB/9 and at least 1.0 WAR. Noah Lowry didn't pitch again after 2007, so we can ignore him. Of the 25 from 2006 to 2014, they averaged 2.06 WAR. The next year, those 25 saw their WAR fall to an average of 1.41.
That's not all. While the group at large did see their walk rate fall off by about half a walk (4.82 BB/9 to 4.35, because regression always wins), their strikeout rates fell more precipitously (8.13 K/9 to 7.30). And their WHIPs, by and large, held steady, but the direction they did go in was bad (1.48 to 1.51), and that despite improving their walk rates. Wins are unpredictable, so I didn't even track them, but in general, the year after their awful walk rates, these guys became worse fantasy contributors.
That's not to say a bad walk rate is ipso facto a harbinger of doom. Included in the list of 25 pitchers is, among others, Clayton Kershaw in 2009. Jimenez, Yovani Gallardo and Matt Moore are still chugging along as starters, while Andrew Miller is maybe the game's best reliever (though he started in 2008). But the list also has Sean Gallagher, Adam Loewen, Daniel Cabrera and Doug Davis. Of course, Jonathan Sanchez makes an appearance.
In and of themselves, walks aren't necessarily a harbinger of doom. But man, it starts you out behind the 8-ball. Immediately, your pitch counts are going to be higher, and your WHIP, too. More baserunners obviously leads to greater possibility of runs scored.
Which brings me to the 27th guy from my sample. Carlos Rodon. A year ago, in 139.1 innings, Rodon struck out 8.98 batters per nine innings and managed a 2.1 WAR. But he also walked 4.59 batters per nine, one of only three pitchers in 2015 with at least 4.0 BB/9 in 100-plus innings (Michael Lorenzen and Trevor Bauer were the others, and Rodon's rate was the highest). From my sample, the only pitchers who bested Rodon in K/9 and WAR were Kershaw in 2009, Gallardo in 2009 and Cabrera in 2006.
Not all walk rates are created equal, of course. In the first half of the season, Rodon's BB/9 was 5.56, compared to 3.70 in the second half. That speaks to the idea of a rookie pitcher figuring things out as he goes along. But it's only an idea, not a certainty; he also had a 4.41 BB/9 in 34.2 minor-league innings before reaching the bigs, and a 3.05 in college, when you'd think a guy with his reputation could dominate.
Rodon came into his professional career with elite pedigree; scouts called his slider the best breaking pitch in his draft class. ESPN's Keith Law said the only thing that would hold Rodon back was (you guessed it) his ability to locate his fastball.
There are two directions Rodon can go. The general consensus is that, at least to a point, he corrals his control, gets the BB/9 under control. He'll probably never be vintage Cliff Lee, but a BB/9 that starts with "2" instead of "3" or "4" changes his prognosis dramatically.
The other direction, though, is that one that held up the guys like good ol' Jonathan Sanchez. Every year, some fancy guy (usually me) would dive on Sanchez, sure this would be the year he gets the walks under *ahem* control, the talent manifests as a true frontline starter. I think Joe Sheehan still wants the Royals to sign him to a long-term deal. Sanchez, though, never did get things sorted out (though he is currently in the Cincinnati system, so ... hope springs eternal, I suppose).
The pedigree and scouting reports would indicate the good future for Rodon is more likely than the bad. But for our purposes, the question is whether that's likely to happen in 2016. It took Kershaw multiple seasons, for one, as he had walk rates in the mid-to-high 4's his first two years and 3.57 in his third before he became the ridiculous figure he is today. Jake Arrieta was a control disaster until he got to the Cubs.
Sometimes, it takes guys time to master control. I wouldn't be at all surprised if that is true of Rodon. Come 2018, 2020, whatever, I can see a Carlos Rodon pulling in Cy Young votes. For 2016? I see a mid-range fantasy starter who strikes out enough guys to be exciting, but walks enough to frustrate you and keep you from running away with WHIP and the like.
In other words, Rodon will be a big help to some rosters. On others, his faults will mess you up. He won't be good for every fantasy team, but the ones he will be good for? He'll be really good.