As I've discussed before, there have been issues in translating studies done in cells and mice to useful therapies for humans. Part of that issue has been due to the fact that many researchers will only use male cells and animals for their subjects. When they try to translate these results to humans, including women, there can be unexpected results.
Now, the NIH is requiring that researchers who receive their funding also include more female cells and animals in their studies. Of course, researchers who work in sex specific diseases such as prostate cancer will not have to comply. Hopefully they will find that their findings will apply equally to both sexes. And if not, we will have this knowledge ahead of time before we try it out in humans.
ETA: A couple additional thoughts now that I have more time. Also now that it got shared (Hi Groupthink!).
Using both male and female animals is not only good science, but also important for animal ethics. We can cut down on the number of animals we breed if we make sure to maximally use all the available ones. Personally, I find it really sad to breed these animals for the purpose of research, and then euthanize the "extra" ones.
Finally, the biggest issue will probably be enforcement. The Nature article states that the NIH will be checking for compliance through data-mining (how specific!) and will also be encouraging journals to favor sex-balanced research (if they listen, that will be very effective). If the NIH doesn't add some bite to its bark and actually revoke funding, it will probably just end up being another placating piece of policy that doesn't bring about any real change.