I think it's kind of like legalising prostitution (heh...let me explain..):
Obviously in an ideal world all contraception would be 100% infallible and nobody would ever have an unplanned or unwanted child, but this is never going to happen. 'Backstreet' abortions are incredibly dangerous and no woman should have to go through them. They often result in death for both mother and baby. Basically, if a woman decides she must have an abortion, it is going to happen somehow, and the reason it needs to be legalised is so that it can be regulated. This means it can be done in safe, sanitary conditions with the minimal amount of risk and distress for the woman.
If it is made illegal and the woman/girl really has to have one, (and there could be all kind of situations, such as fear that her parents will kick her out if she still lives at home, or that her boyfriend will dump her or something, there could be anything) then she will essentially be putting her body and her life into the hands of a criminal, which nobody should ever have to do.
The reason I say it is like legalising prostitution is that that is another unpleasant truth that will always go on, but if it were legalised it could be regulated, and it would be illegal for the women to suffer abuse from their *pimps*, or not use condoms etc . If something is going to happen anyway and cannot be supressed then we should at least make efforts to make it as safe and well regulated as it can be. Alcohol can kill you easily, but when it was made illegal it caused huge crime waves . At least now (although I'm doubtful about how much effect it has) we can try to educate people about drinking responsibly, not drinking and driving etc, rather than simply pretending that it doesn't exist.
Abortion, for me, is a completely impossible issue to come to one conclusion on, I worry about too many people using it as a method of birth control, but I think it is far too complex an issue to simply ban it completely. That would not make the problem of unwanted babies go away, that is merely a symptom of the actual problem. Technically I am pro choice, but if I got pregnant accidentally I'm not sure I could bring myself to have an abortion.
Finally, I don't remember exactly what the rules for abortion are in the US - in England they are legal but only during the first trimester (thats the first 7 weeks i think?) - is it the same for America?