More worryingly, what kind of incentive does this set up? Obviously, it would force companies to adopt policies that overall reduce the reproduction rate of society. This can hardly be said to be a good thing. The number of children my wife and I have should be determined by our ability to support them and our willingness to have them. I am perfectly able to support children with my salary, and I'd prefer to spend my time making that salary, rather than having it handed to us.
Nevertheless, the main reason I am against them is that it removes all incentives from employers (and government, by extension) to make things better for parents that are actually working. I'd rather have a more flexible schedule and more vacation than one year of parental leave. Or to live in a society where only one parent has to work.