The Star Trek [Writer's] Guide (third revision, April 1967--so, end of 1st season, but it may be a section that was in the earlier revisions) says in the Some Questions and Answers section:
Is the starship U.S.S. Enterprise a military vessel?
Yes, but only semi-military in practice -- omitting features
which are heavily authoritarian. For example,
we are not
aware of "officers" and "enlisted men" categories. And we
avoid saluting and other annoying medieval leftovers. On
the other hand, we do keep a flavor of Naval usage and
terminology to help encourage believability and identifica
tion by the audience. After all, our own Navy today still
retains remnants of tradition known to Nelson and Drake.
(emphasis added)
I recognize some rating-esque terms slipped into the show, but let's face it: out of dozens (hundreds?) of named and ranked characters on the show in 100 episodes between TOS and TAS, we can name less than ten who have grades instead of officer ranks.
I'm pretty sure, based on that, the guide, and TMOST, this was the intention, a handful of aberrations aside.
Roddenberry wrote that the ranks in Starfleet are more akin to job descriptions, so it's more your level in your job less than an a military rank. In a letter to Nick Meyer he admitted that they were not always consistent on these details due to the pressures of getting the show cranked out. He also said that the number of people on a starship with purely military functions (weapons crew, etc.) was comparatively tiny compared to the number of technicians and scientists and that Starfleets primary job was scientific and that military type functions were on an as-needed basis. In sort, the ship was mostly astronauts who could pick up a weapon as needed as opposed to soliders who did science as a secondary concern.
Exactly.
(which is why
Enterprise security officers are
so bad at their jobs...

)
There's a wrinkle. I just checked Franz Joseph Sheet 2 of 12, where he lists out and enumerates the entire crew. He says, "The standard ship's complement is 43 officers and 387 crew members: Total 430."
But the "crew member" positions are all listed as ensigns. So apparently FJ didn't realize that ensigns in the Navy are officers. Or else, in Starfleet, they are not considered officers.
At the very least, they're not considered "enlisted men" (q.v. the section I quoted above).
Which is ultimately what I'm concluding. TOS has "officers" and "crewmen" as roles, but their rank structure is simply O1-O? with no E1-E?