I've seen all that happen but I've far more often seen it not happen. I've seen and been in recon units and sniper crews populated only with 'selected' volunteers and been in others populated with whatever the pipeline fed in the way of replacements - to include 'school trained snipers' who couldn't use a mil formula or calculate windage.

Very little difference in performance. In fact, the 'selected' crews tend to be mediocre in performance and showboat prima donnas if you don't watch 'em. What you postulate is true a few places in peacetime, in a busy war no Army could afford that mentality. No decent Commander would tolerate it. Yes, I know all commanders aren't decent -- but most are.

Any good Cav NCO can train 80% of the average grunts to be decent scouts; he can train about 20% of them to be superb scouts -- and those same percentage are about the number that will be decent and super Infantrymen, just different skillsets and yes, the Scout does have to know and do more, a bunch more -- most of these kids can do far more than too many officers and NCOs are willing to let them do.

A decent sniper can train any guy who has a flair for shooting and the patience for the job to be a sniper. How good he will be is a matter of time and experience.

The above comes from about 11 years, four in combat in two different wars, of recon from FMF and Corps to Infantry Battalion level, Cav and Infantry, Mech and Airborne. You show me a unit that acts as you say -- and, as I said, I've seen and been in a few, I know they exist -- and I'll show you a sorry unit (They were and to make your day one of 'em was Airborne ). Good units don't have those problems. I've also not seen the shun idea -- in most units I was in or familiar with, there were people in the rifle companies who wanted to be Scouts or Snipers and had to wait for a vacancy to move...