Secondly: no, we don't have evidence that this might decrease the danger for pedophiles to act on their desires, since the technology is rather new.
Of course we should not enable urges like that. Yet, we have to be realistic: there will always be those that can't be treated. Do you want those who cannot be stopped from indulging in their desires to do so on children's images by real, abused children, or do you want them to vent on made up images?