Not just cabin crew, a lot of the time anyone flying standby is offered the jumpseat if there are no other seats available out of courtesy. Especially if they are an airline employee, but often non-employees too.
My dad was an airline pilot. Policy was you had to be in uniform to sit in the jump seat, and, yeah, it's not open to just anybody. If he was flying standby to get home, he would take it if no other option was open.
