(Seriously, SMM serves either bizarre ILO features that high-end vendors like but are rarely used, or security agencies looking for a layer to hide in.)
Several Intel chipset generations require certain register writes on shutdown (disable busmaster) or they won't _actually_ shut down. Operating systems aren't aware of that. (https://github.com/coreboot/coreboot/blob/master/src/southbr...)
UEFI Secure Boot requires "authenticated variables", which can be updated by the OS (after checking authentication, using a signature scheme). UEFI code resides somewhere in memory, so the OS (or ring0 code) could opt to bypass the verification and simply rewrite those variables. The recommended (but not required) solution is to move variable update to SMM. (https://firmware.intel.com/sites/default/files/resources/A_T...)
Several hardware features are actually implemented in SMM. I've seen SMM-based handling of certain special keys (eg. "disable Wifi" button) where ACPI grabs the event, then traps into SMM using a magic IO port.
Some implementations it'll really screw up any RT plans you might have...
There are also a number of shortcuts for "shutdown is just two writes to a given register" that some OS probably expect to be around these days (a field in FADT) that I'm not sure how a complex ACPI shutdown routine would fare in practice.