Just like a spare tire that you need to check if it has air - you also need to make sure that your backup is working and you have the fuses for it. So if you look at touring setups. they usually have 2 heads and that backup is always ready
So yes get an ELF, TCBAM, MB200 etch - But during sound check, sound check the back up as well and preferably set the output and if applicable the DI level just like your main head.
The one band where they use backline, the requirement for a hot spare backup bass head was in the rider.