A group of cells does not make a human being. If it did, why don't we lament the deaths of countless dead stem cells that get flushed out during menstrual cycles?
True. This is my internal response when [Christian] people talk about life beginning at conception: "An awful lot of people are dying before they are born, then. More than the Nazis ever killed, more than all wars have killed combined. Thanks, God. Reproduction doesn't sound too efficient a process."
I am not sure if it was Dawkins who brought it up or not, but the human mind is developed to focus on a particular microcosm of the universe. If things get too big (thousands of light years, even ONE light year!) or too small (microscopic or smaller, including genes), we can't really grasp them; we just understand the "normal-sized world" that we live in and act in, that impacts us directly and visibly.
So our philosophy/theology is built on that middle-ground aspect of the universe, and we fail to really see what is going on in the larger or smaller scales of things.
As far as "blurring the lines between human and animal," I think this is only offering proof that the lines were already blurred.
Genetically, there's not a ton of difference between us and some of the primates, and not a ton further to some other species. If we can actually merge them successfully, then obviously they were close enough to begin with.
Still, it's more the attitude I think we need to guard against -- that human life in general is something to be tinkered with and used as a commodity and raw material for creating other things. It's less direct negative impact, more detrimentally attitudinal in nature, the things I'd be afraid of.