Abortion should be legal for one reason only - Making it illegal would cause more harm than good. People would continue having abortions, but it would be yet another massive government police state problem, like the alcohol prohibition, or any other prohibition the government has tried to force on people. Perhaps if there were, as there someday may be, real alternatives to abortion. And I don't mean adoption. That seems reasonable to a lot of people, but there
are too many who simply could not get through that, either because they want to conceal the whole thing, or because they believe that killing the baby would be easier than trying to give him or her up after they are born. Someday, perhaps, either birth control will be so perfect and accessible that unwanted pregnancy is almost unheard-of, and/or a baby could safely be "adopted" when only a few months old, using some high tech transplant process, et cetera. But until this happens, it would be like banning anything else; a failure that creates more excuses for oppressive government.
Unfortunately, this is the least-used argument by the abortionists. This is probably because so many of them are so fond of prohibition on almost any other subject, other than abortion.