Gnu, well, I think you are right that a jury might mis-understand dna evidence, and I'm sure they have, and will again. But I think that's the trouble with juries, not with evidence. The reason we have twelve bone-heads off the street evaluate evidence, instead of trained pros who might more reliably understand it, is to prevent government controlling the courts. We sacrifice for that safeguard, and it's flawed, and compromised, like many other things we do.
    Sometimes the very safeguards of defendent rights backfire. I read about a man wrongly convicted of a murder, whose alibi--that he was out and about trying to sell a flashlight or something--might have been more convincing if it were admitted that he was a junkie. As it was, it sounded desperately absurd.
   I'd volunteer DNA without any arrest, not because I don't have any paranoia or because I utterly trust the government, but because there's a certain tiny degree of citizenship I'd weigh against it, and I don't care to be ruled by fear of the abuses I'd despise. Sure, abuses might occur, but in that case, I'll still give them my spit.