If the USB port is broken it would make sense to keep all files that you can on the SD card and only keep on the unit files that cannot run on the SD
such as voices etc. It is not an ideal situation to have a nuvi with a broken USB but workarounds can be made.
If the USB is really broken you cannot flash in pre-boot mode. So if something happens to the Nuvi that needs a pre-boot flash the the Nuvi is Toast.