Why a better plot? It's because I feel like really so bad for Angel Dust, he just deserves to be able to turn into a ball at will.
This way, anytime someone would take advantage of him, he would turn into an invincible ball in less than five seconds and nothing and nobody could hurt him anymore.
So it seems like Vaggie is very surprised to see Angel Dust in his ball form, because she never saw the beautiful spider like that.
It's common for Angel to stay in his ball form most of the time, because it keeps him protected, warm and cozy.
He loves to roll, to bounce, to flirt and to be adorable and cute in his ball form.
He also loves to sleep as a ball, near his pet pig Fat Nuggets.
So many advantages for this cute spider-demon.