Based on what I've read, I think it happens. I don't know why they do it. The aliens may want to study us, they may want to use our DNA to make hybrids or modifications to their own DNA, they may want to modify the DNA of their abductees or their progeny, they may want to steal fluids or organs to help them procreate because they can't do it themselves anymore, they may want to take records of our DNA and bodily makeup in case we ever annihilate ourselves, etc. It's hard to say.