Edit for clarity, I'm talking about when someone holds their sword in the grip where the blade is pointed along the forearm, instead of the usually grip, where it's an extension of your arm. I clearly don't know how much general knowledge people have about swords.
Bit of a rant here. It's like the smallest of nitpicks. It's not a big deal. But I also wish people in charge of how swords in media are handled had to just hold a sword for an hour, because it's immediately obvious how impractical that technique is.
I've had a medium amount of sword handling in my life which is probably why it bothers me so much. Minor amount of 16th century swordplay, 4 years of fencing, half a year with Kung Fu sword form. Not an expert, but enough to know how the muscles work. Holding it backwards puts a huge amount of strain on the wrong part of the forearm, forget blocking or deflecting, shorten your effective range by almost the entire of the sword. You might as well just be punching the dude.
Technically you could argue for some amount of reverse grip. Could potentially catch someone off guard, but also they could just stab you. With a short blade an icepick grip is entirely viable. That's not generally how it's presented, it's depicted as a main character style that implies more skill than the usual plebeians.
It just bothers the fuck out of me. I know nobody cares, and I feel like guns are misused much worse in media. But fuck me it's so annoying every time it comes up.
Oh boy! My wheelhouse!
Like you said, with most swords, and any of the ones you'll actually see in movies or media, it really isn't good. I won't say it's utterly useless, but it's super niche, and won't be useful often enough to merit training that way except for some kind of mandated testing in some specific style. And, imo, if the style is insisting you spend time on it, you're at a shitty school.
In any kind of sparring, the only application would be when after being disarmed and regaining the weapon as fast as possible, and using the position to shift to another while defending. In other words, when you have absolutely no other options. Happily, I've never had to fight with a sword, or against one, because damn. Very few people have, so anything else is armchair hypothesis.
It isn't something that bothers me, as I can head canon a reason that will let me keep watching whatever it is, but it is bullshit. I care too, just not as much :)
Now, knives? A reverse grip has a ton of potential. It isn't the default that movies and TV make it seem, but it has solid applications, particularly with knives that can take advantage of the position. If nothing else, it allows the possibility of a non-apparent stance, and that's a big advantage if you're hoping to avoid any kind of conflict (which is goal #1), but face the reality that you only have 50% of the say in that.
But knives are not just really short swords. There's a totally different set of body mechanics involved, and different tactics needed to be effective. There's some overlap, particularly when you factor in the similarities between fencing and knife training, but there's a lot you just can't do with most swords that you can with knives, and vice versa.