I need you to clarify what you are referring to as "back-up" fuses.
In my experience this is a term used by fuse manufacturers to sell more fuses. The NEC allows the motor overload relay (OLR) to provide running current protection to the motor. The NEC allows the branch OCPD to be sized only for short circuit currents if you are using an OLR.
If you don't trust the OLR (there are lots of reasons not too), you you can size the branch OCPD small enough to also provide all/or some running current protection, thus providing "backup" protection to the OLR.
