But they are both privates. There's no need for inventing two ranks of private there. Technically, the two have to intimately know the details of each other's service years in order to make the right decision about which should lead, as there is no decoration on their uniforms to establish that. In practice, they do not know that, and rely on somebody with more brass to tell them what to do. Why make that "sergeant" when "lieutenant" will do just as well?
But there are staff sergeants, sergeants, corporals, specialists, lance corporals, PFC's etc. in the platoon beside the privates, with correlation to levels of responsibility. Again, it's not the lieutenant and everyone else.
That's where Starfleet "indistinct masses" might differ a bit, if everybody is multitalented and flexibly employed. But that shouldn't affect things much, as said flexibility would in turn require the discarding of rigid microcommand structures, balancing the problem of nobody having microspeciality badges on their sleeves.
Nobody said there should be "microspecialty" badges. What I was asking was: Since Starfleet has obviously decided to grade enlisted personnel as "chief," "first class," "third class," "grade four" etc. what is the downside of recognizing those gradings with insignia?
That depends on what "employed" means. Using the better electrician to make decisions over wiring is sensible, but arguing on that basis that the better electrician can do the requests to the quartermaster is counterproductive - there's likely to be a better logistician in the team.
"Employed" means performing a job function. In response to the idea that there would be "no functional differentiation" between enlisted personnel of all experience levels. If there is no functional differentiation, why does Starfleet give personnel differentiated titles?
That's why meritocracies don't make for good hierarchies, unless one talks exclusively about hierarchy-running merits.
Oh please. Meritocracy means one is advanced based on ones own demonstrated ability, generally used in contrast to factors like inherited social status. Most modern militaries use meritocratic bases for promotion. The "hierarchy" element comes because of the practical requirements of military discipline. Which still obtain in Star Trek's day; there have been several episodes involving who takes over in a crisis, ordering personnel to their possible doom etc.
Third, even if there is no "special command training," which we can't rule out, qualities of leadership could (and most likely would) be a major factor for which personnel are advanced to higher grades.
Exactly. Which is why O'Brien shouldn't be allowed higher grades unless he can demonstrate qualities of leadership.
But he's already been allowed a higher grade: Chief Petty Officer or Master Chief Specialist or whatever. The fact that the production eventually had to come up with an (one-off?) insignia for him seems to be implicit acknowledgment of the weakness of the earlier lack of insignia.
Luckily, we see Starfleet doesn't work that way: two Picards, with identical graduation dates, can make completely dissimilar rank progress in the same number of years, depending on which sort of experience those years bring. Supposedly, the Picard that got higher rank gathered command-related experience specifically.
Abandonment of up-or-out personnel policies is a different issue from acknowledging various grades with uniform insignia.