Pitching v. hitting prospects

Nate Silver has an interesting piece in Baseball Prospectus today in which he argues that hitting prospects are more likely to improve than pitching prospects.
"One thing that distinguishes young hitters from young pitchers is that young hitters can pretty much count on making steady improvements from the time they start playing professional ball until the time they’re 26 or 27.
The same is not the case with pitching prospects. Although there are a few categories of pitching prospects — particularly guys with good stuff, high strikeout rates and highish walk rates — that tend to improve more often than not, in general there is no systematic pattern of improvement after the age of 21 or so.
Sometimes guys get better, of course, and sometimes they do so in a hurry — but you can’t take a young pitcher in a vacuum and expect him to improve the same way that you can for a hitting prospect.
Young pitchers often take less time to become dominant big league performers. Pitching, somewhat contrary to the mad genius reputation of pitchers like Greg Maddux, is more of a purely physical skill and less of a learned behavior than hitting is. Pitchers like Francisco Liriano and Jered Weaver and Cole Hamels — these guys weren’t just holding their own last year, they were among the very best pitchers in baseball. Someone like Philip Hughes might very well be as effective today as he’s ever going to be, before he’s had a chance for injuries and mileage to accumulate. Keeping those guys down on the farm is not conservative — it’s a downright irresponsible way to run a ballclub.