Jackson is probably not a very good option. Almost always successful relievers have their success because they have a dependable "out" pitch. Jackson is a guy with a decent array of pitches he can mix up in a starting role. He doesn't have a devastating slider or change up, no overpowering heat that he can throw 98-99 MPH fastballs for an inning. The odds are that if he can make the majors it will most likely be as a #4 or #5 starter or in long relief roles where mixing up pitches smartly would make optimal use of his skills.
Cashner was drafted as a reliever, and it was only a year ago that the transformation to starter began. As it was, he only made 24 starts between A & Double A ball, pitched only 100 innings--which is only 4 innings per start on average--and he only had THREE WINS all season.
This year, repeating Double A, he had 36 IP over 7 starts before being promoted to Des Moines, where he has pitched only 19 innings in three starts. There is a troublesome caution light on that Hendry probably should not ignore: Cashner is not going longer that six innings in his minor league starts; and he may need to remain longer at Iowa stretching out his starts.
So IMO you are right on target with your concern that in an erratic, hasty decision that the Cubs will send him back to the bullpen. Now admittedly it is not clear that Cashner has enough in his arsenal beyond his high-velocity fastball, and the evidence from his brief minor league starts doesn't necessarily mean that his other pitches have come around enough at this point to succeed in the majors as a starter. But it seems foolish to curtail the experiment: for the long haul the Cubs are going to need top-of-the-rotation SPs after Lilly, Dempster, Zambrano, Silva are gone--and there are no highly regarded #1 pitchers in the pipeline; and with the young Marmol as a future fixture as the closer, it strikes me as shortsighted to convert Cashner back again as a temporary stop-gap in the bullpen of a team that isn't going to contend this year or next.
