Assuming I'm close on your previous question, I'm guessing Rove doesn't have anything particularly incriminating to add to Fitzgerald's case. In other words, he thinks he has a solid case without Rove, and by not calling Rove he may be able to lay a big, fat trap for Libby if he decides to testify by placing some of the discovery off limits. It will certainly make the defense a lot more uncomfortable about calling either Rove or Libby to the stand.
Remember, since Rove is unindicted and has no deal with the prosecution, defense impeachment of his testimony will be difficult if he is called to rebut Libby. Libby cannot afford having a "clean" witness like that providing testimony that substantially differs with him on even small details - if they jury thinks Libby is lying about anything, I suspect he will be on his way to a conviction. [emphasis mine]
I hadn't considered that Rove might likely be called as a witness by the defense. Since Fitzgeralds has not named him as a potential witness, he does not have to provide any discovery on him to the Libby team. So they have no idea what Rove actually testified too, and what other evidence Fitz collected on him. Rove has no deal, apparently with Fitzgerald. So if he is called as a witness by Libby, he has to be very careful not to conflict with this statements to the Grand Jury, or he will have perjured himself. It will be a real crapshoot for Libby to put Rove up there if Rove is afraid he might conflict with testimony at one of his 5 Grand Jury appearances.
Remember, this is pure speculation based on a very narrow slice of a technical legal argument. It does seem to fit the available facts. Rove might be more valuable to FitzGerald walking around like a potential bomb that will go off on Libby if they call him to support Libby's defense. No plea agreement means he has to live with what he told the Grand Jury.
This is going to be quite an interesting show when Libby gets to court.