I do not think using this would result in narrowing the gene pool. What is being proposed is;
Produce say 20 embryos.
Look at the genetic profile of these embryos.
Choose the one that has no genetic disease, the highest IQ, the required hair colour, whatever.
Implant that one, flush the rest down the bog.
Yes, but if people are striving for essentially the same genetic outcomes over and over due to some fetish or deep desire for some specific traits, these will keep reappearing in the gene pool over and over. The embryos people want could be so similar genetically that there is virtually no difference between large numbers of embryos. It just sounds way overrated. What today may be the perfect genes for immunity against West Nile Virus could the next day be worthless against a different strain of the virus, or any multiple viruses or diseases. Genes are often interrelated and are connected and it isnt always as simple as AAG or GTA coding for blue eyes. Genes can be a part of another nucleotide sequence, and thus alteration of one gene would mean the alteration of the other. But if scientists can find ways around these obstacles, I guess people will still be eager to have their "designer babies".