"So if you were attacked and some one emptied a barrel of gun parts in front of you and said , "there you go," you would probably say " better than nothin""
True, but thats not how our system works.
You are right, but at the end of the prosecutions case, a Defendant is allowed to submit his own evidence. If the evidence submitted by the proseci=ution is fabricated, but it cannot be attacked by the defence as in this case, the Defence has no alternative but to lead its own evidence.
Af course the burden to prove guilt is still on the prosecution, but a dishonest prosecution often cannot be revealed from pat dishonest evidence of the prosecution. Defendants often must get that themselves, to introduce the doubt which will prevent the burden of proof needed to be discharged by the prosecution. Defendants can call their own evidence, to attack the evidence of the prosecutors, and in this case, given the resources, that should be fairly easy.