DNA is quite often used to exclude people from crimes, as well as including them, so it could be positive from that perspective. There are a few horror stories around of being falsely accused by DNA evidence though.
You have a very good point, imagine where crimes would be at these days if it weren't for fingerprints and DNA. HOWEVER I am personally happy to give my DNA and fingerprints to rule me out of a crime however I would want them destroyed after the current allotted time under the Protection of Freedoms Act 2012, even in this act the government have given themselves a get out clause that they can keep them indefinitely if they are a terrorist. How do they define terrorist, is it someone who has admitted to a crime of terror is it who they believe, because falling under the Terrorism Act 2000 police officers stop and searched over 100k of people by misusing their power to call anyone a 'terrorist'.
This war on 'terror' is flipping ridiculous, we haven't had a terrorist attack in years, we don't need laws to make it so the police can stop and search anyone they want because if someone wants to commit terror their not going to be stopped by being searched in a street.