Because too many humans will have psychotic episodes when diurnal cycles are taken away?

As Orson Scott Card once pointed out in his guide to writing fantasy and science fiction, any real captain and his officers who acted like the ones on these shows would have been court-martialed and expelled…

Which was something that the character of Riker and his introduction as the First Officer who will forbid a Captain from going on an away mission was meant to solve… but then he STILL takes most of the bridge crew. (“Hey, Ensign Nobody, what’s your duty assignment this shift?” “Oh, I’m to stand at the back panels playing Galaga until Commander Riker calls all the senior staff on an away mission. Then I get to sit at ops!”)

Of course, Trekkies will tell you the books aren’t canon, so make of that what you will.

And that goes doubly for the pre-Pocket Books novels like Spock Must Die!

(And this is a problem I could harp on with almost every episode, but seriously: the ENTIRE senior staff/bridge crew — with the obvious exception of Worf — chooses to go on an incredibly dangerous time travel mission? Does that leave Worf in command of the Enterprise, or is there some other lieutenant commander from stellar cartography or something who finally gets to sit in the big chair?)

As revealed in a later Voyager episode, yeah, that’s pretty much what happens. That particular episode started out showing [perpetual] Ensign Harry Kim sitting in the captain’s chair and commanding the night shift (also revealed to be a standard part of the ship’s operations) while Captain Janeway and most of the other main characters were off getting their eight hours of sleep. As Orson Scott Card once pointed out in his guide to writing fantasy and science fiction, any real captain and his officers who acted like the ones on these shows would have been court-martialed and expelled from the fleet long ago for their incompetence, but, well… that’s standard-grade TV schlock writing for ya.

Lt. Barkley reappears for a story in which he needs to overcome his fear of the transporter.

Interestingly, Dr. McCoy from the original series was also indicated to have that same phobia in the extended-universe book Spock Must Die! in which an experimental new transporter (intended to alleviate McCoy’s fears) accidentally created a mirror-image copy of Spock. Of course, Trekkies will tell you the books aren’t canon, so make of that what you will.

“Hey, we haven’t had an episode where Deanna gets mentally assaulted since last year. Let’s jump on it!”

Yep; kind of this series’ early prototype for those “O’Brien must suffer!” episodes of Deep Space Nine.