A question of necessity

There is a rather heated debate under way just now about just how much information the police need to keep on file about apparently innocent people that may, or may not, help in the detection of crime.
Indeed little mention is made that this same evidence might lead to the rejection of allegations against innocent people.
There are two distinct but entirely similar strands to this dilemma. The one is that the DNA of people cleared of a crime is being kept on the police database. Some five millions files in all we are told, of which a significant number concern those who were found innocent by jury.
The other issue is that of what material should appear on the criminal record checks made about people being hired in positions of authority and responsibility over children and other vulnerable people; teachers, social workers and the like. In this case the issue is not only about those cleared of any allegations but others who may have merely helped police with their inquiries.
The problem is that in both cases the public good may well be served by retention of this data. Indeed there are cases where DNA held on file has been associated with someone connected to a crime many years in the past. Since these crimes have tended to be rape or equally heinous offences there is inevitably a strong motivation for the police and others to want this data retained and available. In the same way it is hard to say with any certainty that information held about someone questioned in connection with a sex crime may not help to prevent an unknown paedophile from being employed in a position that exposes vulnerable individuals to risk.
But surely this is an easily solved dilemma. This data, whether evidential or DNA can easily be retained and available for reference and comparison without any actual identity attached to it at that stage. Then, if the investigators find a reasonable ground for suspicion they could apply to a magistrate or judge for a warrant permitting the identity to divulged and matched to the data, and thus possibly resolving a crime.
It seems to me that we are in danger of allowing the quite reasonable argument that freedom and our principle of innocence before the law is at risk is getting in the way of a proper solution. The problem is that politicians may all too easily in the current febrile atmosphere bend to what they see as public will - either way. And as usual the extreme position is not the best one. A little sane compromise is needed, I suggest.