You should always update BIOS because it is 99% security updates especially in the post meltdown and spectre world.
The never update BIOS crowd made up their minds from the old wife's tale 15 years ago --- when they built their own pc and the only motherboard BIOS update changes were BIOS updates to accept more recent CPU's or faster RAM or fix a specific BIOS bug.
Here is the change log of some random ASUS motherboard from 2005. Go ahead and read through the BIOS change log. Yes, if you lived in that world and you didn't have to fix a specific BIOS bug or changing CPU/RAM, I agree 100% that there was no reason to update the BIOS.
https://www.asus.com/supportonly/A8NSLI_Premium/HelpDesk_BIOS/
Now 99% of all BIOS updates are security updates as it is the only way to fix certain meltdown and spectre security vulnerabilities.