I was using fw_all.bin instead of boot.bin for quick and safe testing for a long time. For example, I was flashing RGN files with fw_all.bin used as boot.bin.
The same ideas works with GCD files.

And also this trick only works if the base addresses of fw_all.bin and boot.bin are equal. It's true for almost all nuvi devices (I can't remember any exception), but it's not true for some devices, like eTrex-10/20/30, GPSmap 60CSx, and many others.
And it's only useful if you are not sure that the device have a reliable pre-boot mode implemented in the bootloader. For example, it's useful for GVN53.