Then why, Chris, if the DNA evidence was so insufficient, did the Defense not use that in court? Why did the Defense not argue the veracity of the DNA evidence?
Rather than state "Hey, look, there's not enough valid DNA evidence because the sample was too small and the girls share a bathroom, etc" the Defense, in fact, verified the DNA evidence on the knife with Sollecito's testimony that it got there when Kercher used the knife. In other words Sollecito admitted that Kercher had come into contact with that particular knife, thus, the DNA evidence is valid. Further, we know that Kerver never went to Sollecito's apartment to leave traces of her DNA on the knife.
ETA: In other words, Sollecito had every reason to believe that Kercher's DNA was on the blade. If the DNA evidence was corrupt, it wouldn't matter. Sollecito's testimony puts the knife in play because we know that he admits it had been in contact with her and we know that she never went to his apartment to make that contact.