(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.)
I have an APM (ARM64) Mustang, and this takes a rather different approach, but probably not one you'll think is better. The chip advertises 8 x 64 bit cores, but there's a 9th 32 bit core which runs all the time, even when the machine is powered down (although obviously still connected to mains power). It runs a separate firmware, in its own RAM, but can access the main memory at will and invisibly to the main OS.
One way to look at this is it's brilliant that we can just put a tiny Cortex-M3 in a spare bit of silicon and have it do useful management stuff.
All watched over by hypervisors of loving grace.
How do you know what the firmware does? Is it even possible to inspect it, let alone replace it? It's just another part of the attack surface - not necessarily deliberately, but if there are exploitable bugs in that firmware that can be triggered from the rest of the system, it's another security risk.
Minor quibble: The IME is not Sun's SPARC architecture, it's ARC International's ARC, the Argonaut RISC Core, which has its origins in (of all things) the Super Nintendo's SuperFX chip.