This argument is disingenuous, because many making it wouldn't care if the cells looked like us. They favor abortion anyway. But many pro-lifers are falling into this trap, including former Sen. Connie Mack, who wrote a column in the Wall Street Journal supporting human stem cell "harvesting."
The question that needs to be asked is what makes human life more significant than a cabbage or a dog? The answer is that we are made in the image of God. If we forget that, as we have with abortion and infanticide, any claim that human life is special will be invalid. It is never right to do wrong in order to do good.