[Update the Second: OK, just did some more research on the underlying "Rule of 30." Turns out that "Minor league innings are somehow not the same as major league innings." It's not clear why this is, but it is. That said, the linked article also mentions a piece that Will Carroll did with Nate Silver that seems to conclusively show that it's pretty exclusively a young pitcher issue. Bottom line: Karstens may be tired, but he's at no added risk of injury/long term ineffectiveness.]
[Update the First: I made a stupid mistake. Karstens threw 16 innings in Indy last year, meaning that his total IP for 2010 was 138.2. 152.0 isn't a big increase from that. If he makes 5 more starts, he will be approaching a really significant increase, but he simply isn't there yet. My apologies.]
That's the increase in IP from 2010 to 2011 for Karstens. That's from 122.2 to 152 as of this morning. 122.2 was itself a 12.5% jump over the previous season.
We all know that research shows, pretty conclusively, that overly large jumps in IP wreck pitchers. Karstens isn't young, but he's not old, either - still 28. Maybe 24% and 30 IP isn't big enough to trigger alarm bells (although I doubt that to be the case), but combined with the lack of life his pitches showed yesterday, I don't see how they can responsibly keep running him out there.
Now, I understand all the issues: they want to see what he's capable of if he's to come into 2012 as a rotation member. He wants to prove that he's durable despite being skinny. There's nobody to actually pick up his remaining starts (Aaron Thompson 5 more times?).
But come on. He's had a surprisingly great season. Before yesterday his ERA was at 3.09, and his xFIP is still under 4.00. He's now hit a wall, and it's time to declare victory and go home. Maybe they can skip a turn and give him another start or two on long rest, but taking him over 160 IP strikes me as reckless.