Do Not Abuse

O, it is excellent
To have a giant's strength; but it is tyrannous
To use it like a giant.

Taking DNA samples from criminals is undoubtedly useful when it comes to diagnosing crimes. You simply swab the evidence, do a look-up against your criminal record database and go and pick up the culprit. There are of course ways to destroy DNA evidence on the crime scene -- chlorine in particular -- but generally it should put the fear into criminals who're on a register that they will be caught if they leave a single hair behind.
But that's about using evidence from the scene against people who have shown themselves to be a menace to society and may therefore be suspect: repeat offenders, particularly in violent crime. It is utterly different from rounding up any old person, taking a sample of their DNA and telling them that they have criminal propensities which are held against them for life. This is what's happening in the UK.
Currently the police can use any powers they choose -- mostly under prevention of terrorism -- to stop you, take a sample and hold it indefinitely. So it is that we find that those on the database represent less the targetted group of suspects from the original design, than a reflection of prejudice on the part of members of our police force. Accordingly, 37% of black men are held on the index (versus 9% of white), while there are 24,000 children have their DNA records stored indefinitely, irrespective of even charges being pressed. You simply have to be stopped and your DNA can be taken.
Why should anyone fear their presence on the list if they're not going to commit a crime, is the typical response. There are a number of issues, notwithstanding assumption of criminality being an infringement of our civil liberties. As biometric data is stored on passports and ID cards, the police will be able to track you wherever you travel -- indeed prevent you travelling should they choose, with no justification required. Moreover, this data can be shared, willfully or unwillingly. It may be stolen, or sold to insurance companies vetting your health or other profiles. If you don't believe me, you should see how wide the electoral register or your credit history are spread: political parties in the UK even used credit records to profile swing voters at the last general election.
I'm not trying to sound like there's a conspiracy theory out there where a police state is trying to monitor our every move: for a start the logistics in retaining so much data would be vast. There is nevertheless an erosion of values of liberty.
Slowly, progressively, irrevocably, the police and government request ever more pervasive rights. Each preceding erosion acts as a justificaiton of the next. We will, within 10 years, be carrying ID cards that hold this data and many people will have access to it, but it will do nothing to serve us.