System Management Mode is the feature that does things like getting your fans to spin up when the processor runs hot. Pretty critical when you put it like that. Yes, that requires "tak[ing] control away from the kernel and running some unknown code at an arbitrary point in the execution". We could do away with such things entirely, but then our kernels would look like coreboot code, and a comparatively trivial kernel bug could permabrick your system. Not a very sensible tradeoff!