Oh, I forgot.
People who want to use the fetus/embryo for human experimentation and people who want to use parts of its body to cure other humans think the fetus/embryo has some worth to them. But it is a different type of worth.
Those people find the fetus/embryo human enough to help science, but NOT human enough to be allowed to live. They might say pro-lifers are too judgmental of others, but at the same time they are perfectly willing to do their own judging of the worthiness of the fetuses to live. They are far more meddlesome in others' lives than the right-to-lifers are. They aren't just taking a moral or a religious or a humanistic stand. They are playing God, controlling who gets to live and who must die.
Their sliding scale of value would be based on the uses they can find for the fetus/embryo.
With their sliding scale the fetuses and embryos with the higher value are more likely to be dismembered.
With your sliding scale the fetuses and embryos with the higher value are more likely to get to live.