I assume it tends to look like something is missing because we're used to seeing Boba and Jango, both of whom have girth belts. I personally like girths, regardless of their make (rope, fabric, another larger leather belt, etc). Definitely helps tie things together, and, like Ori said, can help cover a gap that may exist in the armor.
But, really, it is optional. And unless there's a gap that needs to be covered, I don't think anybody would say anything about there not being one.