In handbook 11.15.3:
The acpi.ko driver is loaded by default at start up by the loader(8) [color="Red"]and should not be compiled into the kernel.[/color] The reasoning behind this is that modules are easier to work with, say if switching to another acpi.ko without doing a kernel rebuild. This has the advantage of making testing easier. Another reason is that starting ACPI after a system has been brought up often doesn't work well. If you are experiencing problems, you can disable ACPI altogether. This driver should not and can not be unloaded because the system bus uses it for various hardware interactions. ACPI can be disabled by setting hint.acpi.0.disabled="1" in /boot/loader.conf or at the loader(8) prompt.
In the default kernel configuration file:
# Bus support.
[color="Red"] device acpi[/color]
Why add "acpi" into the defaul kernel setting and also not recommended this?