Why it is a problem...
The problem is not the fact that the DNA is stored, as such.
The problem comes from the following factors:
* The database is not going to be accurate. Profiles will be linked to the wrong person, present several times, have typos in names... The DNA info itself will be unreliable (contaminated samples or equipment, software error, genuine mutations...)
* As the database grows the probablility of hash collisions of DNA profiles increases. DNA matching is very good if you have a suspect and want to check he/she was on the crime scene, but is not reliable at all if you match crime scene evidence against a few million records.
* The belief of the law enforcement and the public will be (is) that "if the computer (the DB) says you're guilty, you must be" because "real" people don't understand concepts like data quality, hashes. The public has also been hammered with the successes of DNA matching but has never really seen info about erroneous matches and their consequences
* Data in a database can easily be changed/manipulated to forge evidence with no trace whatsoever.
So if you're ready to submit your DNA for inclusion, please remember :
*it is only a checkbox away to flag you "convicted", and you'll never know you were until you're in the clink for a petty offence because it so happened you visited a pub last week where a murder happened today, and your DNA was found on the scene...
*it is only a typo away to have your DNA linked to the worst rapist in the whole universe
*it only take the policeman whose wife you're meeting every other day to get you in the very unpleasant experience of being detained for 48 days (or whatever the number is today), go to court, probably loose your job before (assuming the justice system works) being cleared because you appeared as "wanted" in the DB
If, on the other hand, you're not in the db, all that is much less likely.
Furthermore, I would believe that this DNA DB would in practice, because of its inevitable data quality issues, *reduce* the quality or trustworthiness of DNA evidence.