Android - can fastboot be used to backup device?

If you check with our fastboot tag-wiki, and follow up the link to the List of fastboot commands, you will see the answer is NO. Fastboot only has commands to write to the device. A few exceptions include:

• making sure there is a device at all, using fastboot devices
• some OEM specific commands to read configuration values (fastboot oem <option> – note that all those, except for oem lock/oem unlock, options start with INFO?)
• several bootloader specific commands
• rebooting the device after "work done" (fastboot reboot / fastboot reboot-bootloader)

From the CyanogenMod Fastboot Intro, emphasis mine:

fastboot is a small tool that comes with the Android SDK (software developer kit) that can be used to re-flash partitions on your device. It is an alternative to the recovery mode for doing installations and updates.

Wikipedia describes fastboot as a diagnostic protocol included with the SDK package used primarily to modify the flash filesystem via a USB connection from host computer. (again, emphasis mine).

No source speaks about backing up partitions (or data) from the device.

We can temporarily load a custom recovery

fastboot boot <kernel> [<ramdisk>]

then backup system partitions