"does it really hurt for them to have a sample of your DNA somewhere?"
A good point - so let me offer you this scenario.
You get in a cab - you are dropped off at the place that you have asked to be taken to and pay for the fare. Later that day, a squad of C19 burst through your door and point firearms at your head, before whisking you off to Paddington Green secure unit for nasty people where you wait for 6 weeks whilst they investigate.
What has happened is that later in the morning, someone has telephoned in a bomb threat. They happen to see a cab outside of the place concerned, with somone leaving a shopping bag in a bin. So they run a test in a cab (not necessarily the right one), find your DNA (along with several others) and Robert is your father's brother.
They then tell the press that they have foiled a terror threat (they can't give details as that would not be in the country's best interests) - you were innocent, but they "had to be sure". The authorities trumpet that they have foiled a "major terror plot".
One weasily worded apology later and you can go back to your normal life. True, you no longer have a job, a bank account or any credit rating, so you don't really have a life any more, but it was "all in the best interests of the nation" - so that's good. Also, your passport is taken away, so you can't leave the country.
They also forget to update the records, so your DNA sample is now in the CRB database - as a criminal. You are not allowed to get it updated, as they insist that they never make mistakes. You are then on various watch lists, and occasionally find your actions being monitored.
Couldn't happen in this country? Think again.
Just because you are paranoid, doens't mean that they are not out to get you!