Kim and Jimmy's marriage is not technically legal, which means they CAN be forced to testify against each other in the future. Because Kimberly Wexler is not her legal name. When Kim was a baby, her mother used vacuum man to hide herself and baby Kim from her abusive father. Kim didn't know as a kid, but she does now.

Basis for theory: Remember when (during the wedding ceremony) the judge said "Kimberly Wexler - no middle name??" Too random to not serve a purpose later. Also, Kim was up to something shady during the marriage paperwork scene. Huell pulled Jimmy away from the paperwork desk (to ask about rings) at Kim's prior direction. This gave her a chance to conduct the shady paperwork business. And her body language/movements suggest she's executing a secret plan.