In fact, it has been useful for excluding innocent people. The FBI says that, of many test results, that they could never exclude with standard blood markers, nearly a third of those people are exonerated immediately upon DNA testing. Many rapists, because of this, now plead guilty. In essence, DNA evidence is rapidly becoming, in principle, an irrefutable proof of identification. But of course, nothing is ever so simple.
Scientists are a demanding lot, a skeptical lot, a rigorous lot.
And although everyone agrees that this is a spectacular technology, controversies have erupted in the scientific community from time to time over whether it's really being done right. Fights erupt over DNA fingerprinting because it's such an important technology. They have not been about how to do it in principle. They have been about how to do it in practice, and how well-regulated the practice is. For example, DNA fingerprints should look like bar codes.