Pregnancy has a protective effect on the breast because the process converts undifferentiated, immature breast cells into differentiated milk cells. For a really good explanation of the biology of this, see Dr. Joel Brind's website at www.abortioncancer.com
As for women having less abortions because they are on the Pill, you'd think that would be the case. I did, but I recall reading some studies (sorry can't recall where-I read a lot:) that said many women on the Pill have had abortions. Now whether they were placed on the Pill after the abortion or if it was a Pill failure, I don't know.
The benefits of estrogen may be more than the menopausal symptom amelioration, including protection against osteoporosis, which actually contributes to many deaths since the mortality rate is high in the elderly who suffer hip fractures.
Additional benefits may include improvment of mood including for some women mood disorders that interfere with normal life functioning, protection against cardiovascular disease,and protection against dementias.. The final answers are not in yet.
Having said this, I agree that most of the time estrogen replacement is not a good idea and, were I a woman, would decline.