Here's my argument: in most jurisdictions, a killing of a pregnant woman brings harsher sentence than a killing of non-pregnant woman. In some, it just doubles the crime (double homicide, double manslaughter, etc.) Judging from this, we as a society see at least some value in the life of a foetus.
The issue is a complicated one. We seem to believe fetuses have the right to be born. We also seem to believe that women have the right to their bodies. In the case of abortions, these two rights cannot be achieved at the same time. These are "competing" rights. And while the question of "which right shall prevail" is a classic one for the courts to decide on, people will always go with what they "believe" is right, and disregard the opposing opinion.