Don't get me wrong: on purely ideological grounds, I strongly hope Knox appeals the Lumumba slander charge. I think that she has excellent grounds for appeal on two major findings of law: the issue of mens rea, and the initial (and wholly-incorrect) ruling from Massei that the slander charge should be considered in continuance with the murder (and associated) charges, and thus should be tried concurrently.
My belief is that if Knox were to appeal on these grounds to the Supreme Court, she would stand a good chance of success, and would likely gain a retrial on the Lumumba slander charge alone. And if that were to happen, I think she'd stand a very good chance of winning an acquittal at an appeal-level retrial, since the charge would be shorn of all the circus and bias of being attached to the murder charge, and since Knox's defence team would be able to concentrate exclusively on defeating this charge alone (I get the very strong impression - supported by contemporaneous media reports - that Knox's defence team was putting almost all of its time and effort into defeating the murder charges in the Hellmann trial).
So I would very much like Knox to fight the Lumumba slander verdict, because a) I think it's an incorrect verdict on an assessment of the evidence, and b) I think Knox would stand a high chance of winning a retrial then winning an acquittal in the retrial. My prediction, however, is based on the supposition that - all this notwithstanding - Knox very possibly wants to put the whole sorry saga behind her, and that not challenging the Lumumba slander conviction is a small price to pay for the ability to move on and not have to sit in a Perugia courtroom ever again.
Incidentally, I agree that from what we know, there are simply no grounds for a prosecution appeal on any points of law. That's one of the reasons why I think that wiser and more detached senior prosecutors may decide to withdraw from the arena at this point. But a small part of me thinks that Mignini/Comodi might be able to impose their will to badger the attorney general into lodging an appeal.
Regrettably, I also predict that the statute of limitations may kick in with a re-filing of charges against Mignini in the correct jurisdiction. I say "regrettably" not out of any blind vindictiveness against the man, but because I think it's just and proper that he should be called to account for his behaviour, and that accusations of criminal behaviour should be properly tested in a court of law. Clearly, the Florentine courts believed there was sufficient evidence to prove his guilt beyond a reasonable doubt: it's not therefore unreasonable to want to see the charges tested again.