There are 3 types of stem cells. There are the embroyonic stem cells(ESC), There are adult stem cells, and there are embryonic germ cells. The last category is derived from a fetus' ovary/testicle.
The most useful one is the ESC. This is because when they are harvested at 4 to 5 days, they contain hundreds of undifferentiated cells. Undifferentiated means that the cell has the ability to grow into any type of tissue (muscle, skin, bone, epithelial, etc.).
I apologize to those who already know this. I'm not trying to lecture here.
But I digress from my point. Harvesting embryos is not killing people. A 4-5 day embryo in a test tube is not a person. It has no chance to be a person. If left alone in the test tube it would not make it to term. In order for a fetus to develop it must be implanted in a uterus. Failing that it will not be a human. It is just a possible human.
This failure to implant in the uterus is not common to the "detached" researchers in the lab. It happens on a daily basis to women all over the planet. (Check out this website to learn more about the causes and possible treatment http://www.illinoisivf.com/recurrent-pregnancy-loss/pre-implantation.html)
Conception does not necessarily make a person. The potential for the zygote to become a person does not make it a person. If that was the case then the arguement could be made that masturbation is as bad as stem cell research. Every sperm is a potential person, it just needs an egg. A woman ovulation cycle is wrong too. Every egg is a potential person, it just needs a sperm. Every zygote is a potential person, it just needs a uterus.
"Let the heathens spill theirs on the dusty ground. God shall make them pay for each sperm that can't be found."
-Monty Python "The Meaning of Life"
The semen that you so cavalierly shoot about like Egon Spangler in the hotel scene of Ghostbusters is laden with sperm, and your callous disregard results in millions of sperms' death, but this kills no one. Sperm cannot grow into a person. Nothing can be done to make sperm into a person. If it could, then you'd have people popping out of your old socks and wadded up t-shirts. That's like calling butter a cookie, because there is butter in cookies. Cookies are cookies whether or not they are cooked. It's not just "dough" which is why we call it cookie dough. I would be wary on buying chocolate chip dough ice cream, for I'm kind of a stickler for cookie dough. It would suck to take a bite and taste rye.
Your comparing a zygote to sperm seems careless, considering your screen name. Zygotes do grow into people, and human zygotes formed by human sperm and human eggs are humans. They are not potential humans. Do they have the potential to be anything else? You might say that they are zygotes, but you have to throw the adjective "human" in there to be clear. When people wish to deny the humanity of the unborn, they revert to the use of jargon: an unborn baby is a fetus, or an embryo, or a zygote (depending upon its stage of development). Such semantics are not new. Slavery was called "The Peculiar Institution." The Holocaust was "The Final Solution." And Saved By the Bell was called a "Comedy." A human being can be a specimen, if they are the object of the experiment. This sounds awful, so let's just call them embryos, but let's not even do that, let's call it embryo cell research, but let's not do that, let's call it "stem cell" research and show pictures of brave, noble Christopher Reeve. If we do this, then people won't really know what we're doing. Jews who were thrown into boxcars and sent to death camps were told that they were being relocated for their own safety. Goebbels often equated Jews with vermin, and Hitler himself wrote of them in Mein Kampf as parasites. Such manipulative use of language is absolutely essential in order to dehuminze the victims and turn the masses against them.
The destruction of sperm is not comparable to the destruction of an embryo. Embryos have unique DNA, and it is human DNA. Only human's have such DNA, so an embryo is a human. Embryos live, and their lives are human lives. Purposefully killing an embryo without just cause is murder. I've known (or a least known of) some people who deserve to die (e.g. Screech) but that's because of something that they did. Embryos did nothing wrong. They cannot deserve to be killed, so scientists should not kill them.Just because embryos cannot survive "left alone" means nothing. All babies would die if left alone, as would many old people. This does not make them less human.
It doesn't matter how useful this research might be. It does not justify murder.