Could this angle partially consolidate the 'absorbed by the suit' and the helmetless exarchs in a way?
If you think about it, the Exarch is the name of the original suit, and the gathering of all of the souls of the Eldar who took the mantle in the soulgem of the suit.
The body inside is just a meat puppet, motorised by the Eldar in the stone. Older Exarchs (like the Phoenix Lords) can motorise an absorbed or long gone body in effect, or the suit on it's own as it has enough energy just within the matrix of the suit.
When one becomes an Exarch, as the mantle and suit is taken the soul joins all the others in the suit/stone; the body is no longer of consequence as the Eldar doesn't reside there anymore, but has joined the others in the gem. A 'Younger' Exarch can show the face of the old body (which is just used for the motor functions) while the Exarch himself is the gem.