Abortions must be legal so that any woman who wants or needs one can get one in a safe and responsible environment.
Thankfully, everyone agrees that there are a few cases in which an abortion is necessary to save the life of the mother or prevent serious health issues, and that they must be legal at least that far.
I would hope that most people agree that when a pregnancy is the result of rape abortion should be an option. Rape is an incredibly traumatizing event as it is, and to be forced to carry the physical reminder for at least nine months (assuming that the mother then puts the baby up for adoption) is heartless and cruel. Some women may choose do that path, but it should not under any circumstances be required.
Any teenager should have the option of an abortion, though assuming that the girl has supportive parents keeping the child or putting it up for adoption can be given a little more weight. A teenaged mother loses chances at normal socializing, normal schooling, and a normal life, and no, that is not her "punishment" for making a mistake. Especially because that "punishment" could have detrimental effects on the child, should she choose to keep it, or even should she choose to give it up for adoption: if a teenager is not mature enough or doesn't care enough, she will not be able to properly care for herself or her child.
When a woman is living in poverty, especially if there are other children to care for, an abortion can save her and her family much suffering. No child should have to be raised in poverty, and most programs that would throw money at a mother to get her to decide against abortion will not help her at all after the child has been born - they should at least help her put the child up for adoption. If she can get the care she needs during pregnancy and will be helped to put the child up for adoption, that is a viable option; however, if she decides an abortion will be better for her situation, she should be able to get one.
Some women simply don't want to deal with the hassle, though they can afford to be pregnant and raise a child. This seems like the perfect place to ban abortions, to make people live with their "mistakes" (what if the pregnancy happened due to a failure of birth control?), but even here the mother should be allowed an abortion if she wants one. It's possible that a woman in this situation would simply drag herself through the pregnancy if required to continue it - she would not take care of herself, not stop drinking or smoking, not listen to the advice of a doctor. As in most cases, adoption should be suggested here, but not required. The woman should be able to make her own choice, not have it made for her.
The decision to keep or abort a baby, no matter the circumstances, should be the choice of the mother. There should be restrictions - counseling should be required, in most cases adoption should be fully explored and possibly even pushed, and the abortion should take place before too much time has passed - but it must be available for those who want it. Also, society's views need to change. Not only do we need to not judge a woman for getting an abortion, but we need to support women who decide to keep the child.
You don't have to agree with me, but if you don't, at least realize that the issue is more complex than you might think. This isn't baby killing vs. babies living happily every after. At the very least, realize that there have to be tradeoffs. You can't ban abortion and cut welfare. You can't ban abortion and judge teenage mothers. You can't ban abortion and tell a woman her rape wasn't "legitimate." You can't ban abortion and restrict access to birth control. You can't ban abortion and under-educate your children about sexuality. You get to choose at most one side, though in my mind, it's a choice between two evils.
I choose a third option: abortion must be legal.