Conlanging isn't just gibberish, it's a legitimate artform like anything else.
However, I don't have any issue with using real-world languages to give the desired exotic flair for certain settings. I actually think that was one of the things that was great about, for example, the Avatar: The Last Airbender cartoon, because it took the time to use real Chinese calligraphy as part of the worldbuilding (one of the many flaws with the film adaptation was that it tried to deemphasize the explicitly Chinese influences and ended up with pretty uninspired chicken-scratch fake-ideograms for the writing).
Similarly, the video game Riven: The Sequel to Myst had some cutscenes where the fantasy native population were actually speaking Tok Pisin, which is a creole language from Papua New Guinea. This was only fragmentary, though, and much of the Myst games have a quite well constructed language "D'ni."
Interesting tidbit! I had thought they were all artificial.
But the moment you start using existing language in a new context, you get into issues. You've got words and whole concepts missing that you have to add to something that's more or less complete.
Dothraki needs horse-related words. You need to be able to express certain social concepts that are unique to this fantasy culture.
So you set up to build this words and phrases. You derive phonotactics of a real language and patch it with your stuff. Now you've got new language based on an existing one. So you didn't preserve anything. And if I were a user of the originl lang, I'd be upset you appropriated and twisted it.
I feel similar with history, if I am just doing it for entertainment I'd much rather learn a constructed history rather than the real one.
Whether you take Sapir-Whorf hypothesis for granted or you think that its inverse is true, cultures and languages codevelop.
You wouldn't say "keep them at the bay" in a language used by a land-locked people. You won't call tomato "pomodoro" if gold in your area doesn't have impurites that make it red and not yellow.
You have to understand that language on a very basic level is a construct of a culture that used it. You can't separate them.
And then there are practical reasons. You won't get actors to learn Mongolic language to play Dothraki. Conlangs aim to combine "alien" with "familiar" which is a must in a movie, game or a book.
If you mean to reference the expression “keep someone at bay” (no “the”) with the meaning of holding them back, that comes from “bay” as in “the howling bark of hounds” rather than “bay” as in “a partially enclosed body of water attached to a larger area”. Completely separate etymology (from old French «bay» and «baie», respectively, ending up in words that happen to be spelled the same in English.)
So, a fine idiom for a landlocked people, not so good for a people without dogs.
Why would actors learn a language when they can just learn their lines? If you're already borrowing concepts from other languages why not the whole thing. I don't know if conlangs are better, but I don't think it would be unethical to use an existing ancient language for a movie or TV show.
[1] https://www.player.one/far-cry-primal-interview-how-ubisoft-...
John Kani, a veteran South African actor playing King T'Chaka in the Marvel movies "Captain America: Civil War" and "Black Panther", decided to speak the Wankandan language parts in Xhosa, a South African language that he is a native speaker of. He taught the rest of the cast their lines. I think it sounded realistic.
https://www.thesouthafrican.com/john-kani-influenced-the-bla...
https://qz.com/1192662/black-panther-wakandas-language-is-is...
A lot of those people are proud of their language and would like to see it live and thrive again.
https://reddit.com/r/conlangs/comments/8wjs73/is_there_a_cit...
Bonus points if it's sold as random fantasy language of a strange country and the main cast doesn't understand it either.