I've been against DNA databases for years, but I've always felt that the generic arguments I've been using were a little pallid, shall we say. And now, in what amounts to almost a throwaway comment, the wonderful Reg gives me what I've been looking for:
Although police are keen to bang the drum for cases where DNA evidence has proved vital, there are obvious privacy objections as well as fears that over-reliance on DNA evidence will lead criminals to use it as an alibi - infecting a crime scene with someone else's DNA.
At the moment, there's not much point doing that because DNA isn't regarded as as an indispensable, infallible tool. Put everyone's DNA in a database, and the police are bound to get lazy - that's human nature - using it as a quick and foolproof method for finding perpetrators.
At that point, it will be worth seeding crime scenes with some judiciously-chosen DNA - secure in the knowledge that the rozzers will be able to work out whose it is. At this point, DNA begins to lose its value, as everyone starts sprinkling the stuff everywhere, utterly confusing the DNA bloodhounds.
And so, inevitably, we will be left with a huge DNA database, useless for its original purpose, built at enormous cost, posing an even huger security risk. Great. Not.