Since SMM allows for all kinds of things that ring 0 can't do (one example: writing to flash even if write protections are enabled, unless a second set of write

Since SMM allows for all kinds of things that ring 0 can't do (one example: writing to flash even if write protections are enabled, unless a second set of write protections, added in later chip generations, is _also_ enabled), arbitrary code execution in SMM is a bad thing. Also later ring 0 code can't get rid of code installed into SMM earlier (that may still trigger through events or timers).