The difference, of course, is that some folks are eager to assign different values to the different stages of human life in order to promote a utilitarian argument. By acknowledging that a blastocyst is a certain stage of human development, one acknowledges the possibility that it might have intrinsic value as human life. This is an important point, when considering a utilitarian argument.
One can then go on to argue about the intrinsic value of this particular form of human life, if one wants to. I, however, am of the opinion that this value is ultimately unknowable, and thus must be divorced from utilitarian analysis.
To extend it to your weather pattern analogy, if somebody declared all rainy days of little worth, and got hold of Karl Rove's Weather Machine, and set about eliminating rainy days because he found sunny days to be of higher value, then we're getting into the same territory. The counter argument that a rainy day weather pattern may be as valuable as a sunny day weather pattern would start with the acknowledgement that they are both weather patterns, and both connected to a larger whole, and may both have similar intrinsic value.
If one just says that rainy days are not sunny days and that sunny days are better, one misses the point.
Certainly. But utilitarian arguments are not intrinsically improper. Indeed, I would suggest that they are a necessity.
It is not a particularly easy task to draw a scientific "off limits" line when dealing with pre-implantation reproductive cycles. For example, artificial implantation of blastocysts has some considerable success in generating term pregnancies, which would not occur if manipulation of blastocysts (with the concomitant percentage loss of blastocysts generated and used in artificial settings) was forbidden.
I think we have to realistically recognize the ubiquity of pre-implantation reproductive cell generation as a utilitarian matter, or otherwise consign to the dustbin a great many reproductive advances in both viability and health.