It's not all that different from schools that adopt differentiated learning groups. Test scores, at least, have proved that it is effective. And the they track kids in the the first group too to make sure they are showing good progress year to year, rather than getting ignored since they may be reading (or doing math) four years over grade level and do great on testing. It's hard. And things can change over time (just like in gym).
The high acheivers grouping has a core group of kids all the way through elementary school, with some that come and go, but a bunch of them that are there throughout. Their teachers get specific training on how to challenge and teach them, and also to be aware of social challenges etc. these kids may face. Even though in our school the first group is together in a classroom with one or two other groups, you can bet that by fourth grade all the kids have been labeled as the "smart kid" group or "nerdy kid" group by their peers. I would not want to be a parent of kid who just missed out on being the high acheivement group, if I felt that my child was getting "less quality" teaching or opportunities. It's a balancing act. And all the kids deserve great teaching and the opportunity to acheive to the best of their ability. Differentiated learning is supposed to strive for this for all types and levels of learners. A couple of my friends who put their kids in charter schools or home schooling did it partially out of frustration at this--not getting in the top group and not being happy with what they perceived their kid's opportunity was at that school. The educational equivalent of gym switching...
The reality is that parents are paying out of their pocket for coaching in gymnastics, and it rankles if we feel that our kids are getting less opportunity or quality of coaching, because of course if you get more time/better coaching you are more likely to succeed. It helps when all the kids are really being challenged and getting great coaching, and feel like they can advance into different groups if they progress. That's how I feel at my daughter's gym, and it is just a great feeling of trust for the coaches.
A gym that claims they want the best for every gymnast and that the true measure is if the kids are progressing--well this rings false on the ears of Xcel parents or "B group" parents if their kids are getting a lower quality of coaching, less hours and paying more per hour. In fact, it can feel like they are being bamboozled. On the other hand, if they are getting great coaching and can advance if it is warranted (and particularly if they were all in one preteam group for the first year or two on preteam) and had the same opportunity as everyone else to go for JO), then it "feels" more fair.
The screening of body types at age 4 or 5 for that gym that really just wants to produce a few NCAA scholarships and maybe on Olympian one day... well that might not be the right gym for everyone's kid. Some kids would have a better chance somewhere else if their goal is to compete all the way through college because they might just get overlooked at the "top dog" gym and not given the opportunity to acheive to their potential there. That's why mymeetscores is interesting, but the decision of choosing the right gym is a little more complicated that just picking the one that everyone else wants to go to.