One thing I've picked up from watching various investigative shows (the ones that give the story of an *actual* case, not "Law and Order" type things) is that even DNA evidence isn't an absolute. You've got all the various places it can get contaminated (at the scene, during transport, during testing) plus there's still a good bit of ambiguity even when the collection and testing goes perfectly.
It's another one of those things that for the purposes being put forth doesn't need to be "perfect," but certainly needs to be both far better and far faster than currently before I'd be willing to trust my identity to it...
I remember that actual prosecutors and defenders were constantly bothered by the fact that jurors, having seen these show, actually expected clear black and white evidence. They had to teach the jurors that the real world is not like TV before the case could continue.
And several people commenting here have obviously based their ideas on the misleading TV shows (or cinenma films) that they've seen. That seems blatantly obvious because it's difficult to imagine anyone gettig it quite so wrong if they havent been specifically misled.