I heard Fireflies might not have a use, but what if you could catch Fireflies with an empty bottle, and depending on how many you caught, the bottle lights up more and more, and you can place it on any block, and it will light up the room, much like a torch. And if you're like me, and feel bad for the trapped fireflies, then maybe you need to feed them something, such as honey to keep them lit in the bottle.
This is definitely an idea that's been proposed many, many times, so surely Mojang must be aware of it. Unfortunately, they don't communicate things well, and the community seems to assume that they could've easily just added in the fireflies anyway without understanding what the actual reasons may have been.
I don't think the problem was not having a use for them, but having the time time and limited by the update deadline. Who knows if they were given like another half of a year to work on the Wild Update, they could have had enough time to work on how to implement fireflies, but considering the frog consumption matter with fireflies, they had to utilise the time instead to focus on how else to implement a way for the froglight blocks to be made (that was the main point regarding frogs and fireflies originally). Hence, fireflies became the least priority compared to the other features in that update.
Hopefully, if they find the time, they could add them in, but it would definitely take a while trying to figure out the best way to do so in terms of both usage and performance (if they were as mobs/entities, that could be a performance issue considering the amount of them; if they were as a particle effect, that's an entirely other functionality to try working out). Albeit it's understandable why fireflies didn't end up being added in anyway. However, people have made fireflies mods, at the least.