I hate slippery slope arguments. Too "predictive" of what one thinks "might" happen as a result. Yes, we now work with human cells in labs, but not ones with the potential (well, Natural potential) to become whole humans. But they are alive. I guess the slippery slope argument would be that we will move on to cells with a Natural potential to become whole humans (embryos), and from there, to what?
But the slippery slope argument often does not hold true. We used to experiment on living humans against their will (mentally disabled, Africans, concentration camps), but we don't today experiment on even more people against their will.
Another argument against stem cell research is that embryos have the potential to become whole humans, and current research (not including the new study noted above) requires the "destruction" of an embryo during the harvesting of stem cells. But why is this eight-celled embryo growing in a dish used as the fixed point of human potential? My gametes right now have some potential to become a human. The frozen embryos left over from IVF treatments have some potential.
But, in each case, something must be done to actually allow the cell to become a human. The gamete must be fertilized, and the embryos must be moved from a dish to a living uterus. It's like suggesting that I myself could repopulate the Earth- no, I'd need something else, in this case a fertile female (weak analogy, but it's late).
The argument I've mentioned above ("sentience") has the same flaw- when exactly does sentience form in the developing fetus? But, it avoids issue with stem cell research and first-trimester abortions, and even gives suggestions for end-of-life matters for the brain-dead or nearly brain-dead.