Did you know that destruction of DNA evidence can and often is a part of plea bargains? That would be one way of heading off those pesky Innocence Project folks. Hey, if all you want is a result that will stand forever, that will do it. Grits has a great discussion on the topic, along with some indignant responses from DAs.
One interesting aspect of this is that Dallas County, where breath-of-fresh-air DA Craig Watkins has been busy working to release unjustly convicted inmates, is fairly obsessive about saving evidence from its cases, but the justification of this is more quirk of history than anything else. The wheels of irony may grind slow, but they too grind pretty finely. Eye on Williamson has more.