I think people really really miss the point when they say "well the flyweights would easily beat a common presumably untrained person." This promotion is called the "ultimate fighting championship", it's meant to create "ultimate fighting champions", not "untrained fighting champions", understandably some flyweights walk around a bit heavier, but if you had them cut to 125 like they do before each fight and then walk into any MMA gym and fight the local fighters there, they'd get their butts kicked by all the regional fighters at lightweight or higher. How are those "ultimate fighting champions?" At the very least, combine BW and FLY at the heavier weight and you have a deeper division of small folk.
People who like the division like it because of the aesthetic. At such a low weight, wrestling doesn't typically work well, and fighters don't have eraser power, though they have lower force tolerances (if they fought a slightly higher weight their opponents would not have low force tolerances). As a result, the division is primarily boxing, and it's very technical compared to the higher weights, their footwork is faster and their movement is better, which gives the illusion of competency. But a fight between even a flyweight and a technically inferior lightweight would be a miserable beating.