I don't think anyone would argue DNA evidence is a 'magic bullet' (or I hope not), but it does seem to be pretty a pretty useful tool in a criminal justice system that is a lot more flawed than we might like to think.
There's umpteen examples of people who spend years in prison and are subsequently found to be innocent of the charges levelled against them due to DNA evidence. According to the LA times, 300 people and counting in the states alone have been exonerated in this way:
http://articles.latimes.com/2012/oct/01/nation/la-na-dna-evidence-20121001
Note that many of these people (such as David Lee Wiggins) were arrested and sentenced long after DNA profiling became available to police forces.
How many of those stolen years could have been avoided by use of a DNA database? How many people are there in our prisons whose convictions could (and therefore should) be reversed? How many people are walking around today because the wrong person was convicted?