I live in the Netherlands and have been (casually) following this case, so here's some background that might be relevant:
Some posters here wondered why this guy agreed to give his DNA. There is some evidence that he is mentally ill: in 2009 he was convicted of stealing a neighbor's car and joyriding while under the influence. He claimed not to be in control of his actions at the time, and that he only came to his senses while in the car. A psychiatrist diagnosed this as a dissociative fugue at the time (he was still convicted). A neighbor of his also recently stated that he has mental problems.
So it's quite possible that he doesn't even remember the rape and murder. It's also quite possible that it's simply a lame insanity defense.
I think one of the reasons for this DNA-dragnet was that the area is sparsely populated, and the cities/villages there are tight-knit communities. Many people there are blood-related, so the odds of finding a partial match should be good.
It may also be the reason that so many people were willing to participate; they wanted to believe that the perp was an outsider (some still think so). My guess is that some social pressure was involved as well.
Anyway, I don't see something like this working in the major cities. Very few people would volunteer, and chances of finding a relative of the perp would be much lower.
Also, there were other DNA tests before, these were done to rule out two suspects - asylumseekers from Iraq and Afghanistan, respectively. They had been fingered in an anonymous letter, possibly with a racist motive. Analysis of the DNA found on the victim pointed to a caucasian anyway (not uncontroversial).