It's also a convenient place to put in all the things they don't want to hard wire. Which gets more every day.

Need to maintain crypto keys for SGX enclave memory? Do it in the ME. Need to do some extra stuff on suspend/resume? Do it in the ME. Not sure if any other special handling might require updates at a later date? Do it in the ME. ...

There's no need for nefarious purposes to explain why the ME isn't optional anymore - it's just more convenient.