They're rather suicidal in my opinion. But, I think the damage reflection does not take into account the attacking units armor. So, position the ME against a hard hitting, well armored enemy unit and it'll cause some serious damage in return. The ME dies, but assuming you're getting the ME from a shard you'll get another one in about 5 turns.

My opinion - they are totally useless.

I've found them very useful as free suicide troops to weaken armoured units. Also, IIRC their own defense isn't bad... The Ancient ones in particular seemed very tough for a free unit with such a powerful ability.