ugh sorry i deleted my post because I wanted to rephrase it but yeh, Hoshi is there as a science specialist. With Uhura I'm not sure, since I've not watched much TOS, what her expertise is other than Open Hailing Frequencies (which doesn't in of itself seem to require a comissioned officer). But I had always assumed it was more of an engineering role, like fixing and maintaining the Space Radios.
This kind of feels like the "what does the receptionist actually do?" effect. Most people in an office ignore the front desk staff... and yet, the entire office collapses when the front desk can't or isn't doing their job.
People in charge of communications in a organization spend a
lot of time keeping track of who knows what, recording information, and filtering information. A big part of Uhura's job was surely deciding what Kirk needed to know
now, what he needed to know during the routine meeting at the end of the shift, what should really go to Spock, and what can just get put in the end-of-week report because it doesn't matter. And then remembering what went in that end-of-week report when suddenly it
does matter. This is not unskilled work. And while modern computing systems help a lot, that kind of filtering and prioritisation still needs a human in the loop. Certainly there wasn't a way to automate it in the 1960s!
Plus, there's presumably at least four people doing that job - three watch-standers and a spare - so someone needs to be in charge of them. TOS doesn't have a lot of rank levels, so Lt. seems reasonable. (Arguably, the Yeomen are also under Communications, though that's fan speculation on my part.)
We also shouldn't assume a 21st Century Earth level of coms reliability. In the 1940s-1960s, just being able to get your radio sending a message to the other radio was skilled labour. Who knows what getting two subspace radios to work requires? Especially in the TOS era; by TNG we can assume a network similar to a cellular network covers most of the Federation, but in TOS, we're dealing with future-short-wave radio.
In short: the person in the Communications chair needs a range of technical, interpersonal, analytical, and organizational skills at a fairly high degree, and at least one of them needs to be an officer in command of the rest. TOS may not have been great at showing this, but I'd not be surprised if Roddenberry and Meyer didn't really appreciate how much work their admin assistants did, too, so I'm not surprised that kind of work (which is, admittedly, not dramatically interesting) didn't make it onto the screen.