Without exceeding fair-use limits, it's really hard to find a blockquote for the beginning of this Post-Gazette article that explains its argument well, so I suggest you go read it. But here's a snippet:
Nineteen National League pitchers logged 200-plus innings in 2007, with Snell tied for ninth at 208 and Gorzelanny 17th with 201 2/3. The list included most of the league's top overall performers, with Brandon Webb, Jake Peavy, Aaron Harang, Carlos Zambrano, Roy Oswalt, John Smoltz, Brad Penny, Jeff Francis, Dontrelle Willis ...
Hey, seeing a pattern yet?
Right. With the sizable exception of Arizona's brilliant Webb, most either have regressed this season or, worse, have gotten hurt.
The article then points out that, among other things, 14 of those 19 have worse ERAs this year than they did last year, and eight of the 19 have already missed too much time to pitch 200 innings again this year.
The implication of the article--the author, Dejan Kovacevic, is too smart to say this outright, and he leaves it up to the reader to decide--is that pitching 200+ innings in a year leads pitchers to be worse the following year.
I wasn't too surprised by any of the numbers Kovacevic mentions. Pitchers are extremely volatile, and a guy only gets to pitch 200 innings in a year by being 1) pretty good and 2) healthy. If you take the class of pitchers who are fortunate enough to be good and healthy in an entire year, it's very likely that group, as a whole, will take a big step backward the next year. That's just regression to the mean, and it doesn't necessarily mean that pitching 200 innings, as opposed to a lower number, is the problem.
To show what I mean, let's look at the class of NL pitchers who threw between 160 and 180 innings last year. 160 to 180 innings seems to be a moderate number for a starting pitcher, so if it's true that throwing 200+ innings is bad, this group of pitchers shouldn't have had the same problems the 200+ group had.
Here they are:
Matt Belisle
Paul Maholm
Oliver Perez
Scott Olsen
Braden Looper
Chris Young
Matt Chico
Aaron Cook
Josh Fogg
Kip Wells
Adam Eaton
Chuck James
Of these twelve players, nine have had significant injury troubles since last season ended: Belisle (forearm), Olsen (no DL, but shoulder problems in Spring Training), Looper (no DL, but shoulder surgery in the offseason), Young (face), Chico (elbow surgery), Cook (no DL, but shoulder soreness in Spring Training), Fogg (back), Wells (blood clots in hand), Eaton (no DL, but shoulder trouble in offseason), and James (shoulder).
Belisle, Perez, Young, Chico, Fogg, and James all have markedly worse ERAs than they had last year, and all of those except Young and Perez have ERAs north of 5.50. And Wells has hardly pitched at all.
Olsen and Cook have been downright good. Maholm, Looper and Eaton have been decent. But the fact remains that, as a group, these pitchers are likely to be a bunch less productive than they were last year, mostly because Belisle, Chico, Fogg and James have been so bad. And this group seems no more immune to injury than pitchers who threw 200+ innings.
The point here is that, generally speaking, pitching 200+ innings isn't what's dangerous. What's dangerous is pitching at all. If you repeatedly try to throw a baseball 95 miles per hour, you will probably get hurt sometimes, and there will probably be some times where you don't do it as well as others.
Obviously, I'm not suggesting that a 20-year-old should be pitching 220 innings a year in the big leagues, just that this problem involves all kinds of nuance, and there's nothing inherently worse than 210 innings as opposed to 170. That the pitchers who threw 200+ innings last year aren't having the same success this year says far more about the volatility of pitchers in general than it says about that 200-inning barrier. You can't blame the struggles of Ian Snell or Tom Gorzelanny on that. (It is fair to wonder if Jim Tracy's overuse of Gorzelanny down the stretch last year has contributed to his problems this year, but that's a slightly different question, since all kinds of things could've happened last year that would've allowed Gorzelanny to get to 200 innings without any 117-pitch outings in meaningless games in September.)
This all goes back to something WTM has been saying for years. Spending a bunch of first round draft picks on pitching instead of hitting is, generally speaking, a dumb thing to do. Forget the 200+ inning pitchers for a second and look again at the 160-180 inning group. In general, they didn't improve this year, even though most of them aren't old. And the reason they didn't is because pitching is just really difficult and dangerous, and a lot of pitchers will completely fail in any given year. Any team that aims to build through pitching in the same way the Pirates did is making a house of cards.