The one facet of stem cell research that everyone ignores/forgets is what happens if we perfect it? It will be easy to prolong life - Kidney failure? Grow a new one. Heart Attack? Grow a new one. Drowned your liver in alcohol? Grow a new one....
No one will die ------> Overpopulation
No one will die ------> Overpopulation -----------> People will die.
Many present points with the assumption that we can somehow supersede the laws of nature. We can't. We can for a little while, but it will always come back to bite us. Progress is inevitable in our society; to restrict this one would just be arbitrary. The ethical implications are unconvincing. But I wouldn't worry about one day reaching the point where we're immortal or something. At this point, there are diseases that are horrible, and we can potentially cure them...... very hard to say, "hey, no, the buck stops here buddy, you can't cure that disease." It's hard to tell people that.