Steve Benen drags out a George W. Bush quote: "I don't believe my role is to replace the verdict of a jury with my own... unless there are new facts or evidence of which a jury was unaware."
New facts? About Libby? Do tell! Oh, wait, no, he was referring to the 152 Texans who were executed under his watch as governor. But, you see, they were all dirtbags and it's not like the Texas justice system ever errs. And none of them were friends of his, so it was totally different. Oh, and Bush was really overturning the judge in the Libby case, not the jury. Have we mentioned that obstruction of justice is barely even a crime? Yeah, that's it. No double standards here. Move along...
In any case, this here seems like the most cynical--and so probably the best--explanation for why Bush commuted Libby's sentence instead of giving him a full pardon: Libby still gets to invoke his Fifth Amendment rights, should he be questioned further during his appeals process.