The cut-fastball is a pitch of pretty tremendous use. It's a gleeful compromise for pitchers who throw too straight of a fastball, and/or can't throw their slider for a swinging strike. Normally these are some fatal flaws for a pitcher to have--and they probably should be--but molding them into a hybrid suddenly gives the hurler a weapon that rides in on opposite hand hitters and creates weak contact, but is easier to control than a true slider.
White Sox pitching coach Don Cooper has made quite a reputation for his ability to revive the careers of mediocre pitchers, so perhaps it's no surprise he's been teaching the cutter to a healthy amount of his uh..."projects".
Notable examples are when a new cutter temporarily made Esteban Loaiza an elite-strikeout artist in 2003 and facilitated John Danks' breakout 2008 season. Not-notable examples include trying to teach it to Scott Linebrink.
With the fight for the last few spots on the 2011 White Sox pitching staff being something of a 12-way (or however many candidates there actually were) tug of war between men with broken careers, it's not surprising that a few players got some lessons on the cutter over the course of March.
However, it's interesting to note that the two guys who made it (Humber and Marquez) prominently credited their brand new pitch for their success, and the small amount of buzz Shane Lindsay received, centered mostly around his expanded repertoire as well.
Having an open casting call for bullpen spots amongst a host of minor league discards isn't the most enrapturing thing your favorite team can do, but it appears the Sox know what they want when they revert to these measures, and aren't as much under the delusion that the next diamond in the rough is walking through the door as we might have feared.
These guys don't have much, so Cooper is using a reliable tool for keeping them in the strikezone and neutralizing their platoon splits. We'll probably never see another Esteban Loaiza situation again, but can probably expect the same type of flash and fade as the league re-adjusts to anyone who does emerge.
Journeymen armed with cut-fastballs ain't much, but it's better than journeyman armed with the tools of established failure.