Doesn't work to prevent crime? Or doesn't work too suppress dissent?
There's also the echos of visions that occur which is how the villain manages to get away with a murder, he uses his inside knowledge to copy cat another murder method which allows him to have it written off as an echo.
The wilder question in the movie is how the precogs are randomly created mutations in response to the mother taking a weird street drug during pregnancy. They'd have to dose pregnant women and hope they gave birth to more mutants if they ever want to replace the set they have, and I believe they're planning to roll it out nationwide. But given how crappy the precogs' lives seem to be maybe that is just a cost they consider worth paying.