Via Radley Balko, we have a dude who did ten years on triple murder set free because, well, the state, among other things, destroyed evidence. Mind you he hasn't been exonerated, and may well have did it. But the cops and prosecutors cut corners trying to nail him and now he's on the street.
If you're interested in putting the right people in jail, you aren't doing any favors by being sloppy. This isn't said enough. Destroying evidence isn't a "favor" to the all the innocents criminals prey upon, any more than blowing the rent money to take your kid school shopping is an act of love.