Suspect Mayor Q is right about it encouraging lazy policing. Which is going to be even more of a risk under government plans to slash police numbers and shut down the forensic science service, leaving the work to commercial enterprises that have an incentive to come up with the 'right' results to gain repeat business (and won't be willing to do the more complex/expensive work).
An example, not involving DNA but illustrating the problem with lazy policing. An intruder broke into my sister's home when she was asleep, stole all her undies (from drawers, linen bin and airing cupboard - this guy was thorough), cut her phone wire, took her carving knife and got into bed with her. Thankfully he ran away when she woke up and screamed but still terrifying.
Initial police response was great, house crawling with officers and promises that 'we'll catch him, love'. And we believed them.
They spoke to every man she'd ever known, friends as well as exes, causing considerable embarrassment. Then ran the prints through the national fingerprint database. That was it.
Six months later she got the evidence bags back in the post with her bedsheets - no warning and worst thing was the bags were still sealed so it looked as if they hadn't even been opened for testing.
They failed to follow up a lead from a cabbie who had picked up a man behaving oddly (in a hell of a hurry and breathing as if he'd been running) from the end of my sister's street at the same time this guy would have left her house.
I worry that a DNA database that is growing by stealth will only encourage more of this kind of crappy policing, where you just run someone's data through the records and no match is the end of it. That guy is still out there.