Since Winky was present at the testimony of Bartemius Crouch Jr. (in the Defense Against the Dark Arts office), she would have remembered it. In addition, she could confirm that what he was saying was true. Why did McGonagall and Dumbledore give up on finding evidence for Crouch's testimony after he was administered the Dementor's Kiss?
In addition to confirming Crouch's testimony with her own, she could also provide her memories that would be visible through use of a Pensieve, and along with those of Snape, Dumbledore, McGonagall, and Harry, this would have made for VERY convincing evidence. In addition, who is to say that Crouch lost his memory after the Dementor's Kiss? Do dementors suck out memories in addition to the soul?