CRYING SOLVES EVERYTHING.
Seriously, I love how Sailor Moon shows traditionally “weak” and “feminine” emotions can still be powerful and kick ass, and are nothing to be ashamed of.
In Usagi’s case, she just has to learn how to apply these emotions properly and in the right dosage, they give her strength. She can’t just use ultra sonic waves forever, it’s an attack that has no focus and requires more, but this is also an important and powerful part of her,
I wish they’d have used the sonic waves again at some point. I can’t recall the manga properly but I think even in that it was like only used in her debut and then never again.
Why? It’s a cool and useful ability
It’s used again twice, once in the third chapter and once in a side-story at the dentist with chibiusa.
The attack is uncontrollable and hurts everyone in the vicinity, including civilians, and can even destablize infastructure. Once Usagi is used enough the battle she’s not going to break down and wail in the middle of it, it’s really not needed. In the dentist sidestory, she has a fear of dentists, so that’s the only reason in came out again.