Ok, lets say fusion of a soul shadow essence into a machine is a reversible process. Just read that to yourself. Doesn't sound quite right. Even then, their is no way it is a easily reversible process. Certainly harder then getting into a marine suit right? (which is quite hard apparently). So if Im going to be spending time in a war machine for...months, if not years...Hands would help. A lot.
Even if the face thing isn't 100% practical in war, you have to remember, the protoss are not zerg. They aren't all about 100% efficiency. The dark templars live a highly individualistic and tribalistic warrior culture. Look at the samurai, who dispite being basically the most skilled swordsman in japan, or the world for that matter, wore unpractical armor, filled with heavy metal decorum.
The dark templar wouldn't give up their individuality, their face, and basically their ability to feel like a protoss...to save material, because just because they have a face doesnt mean that shooting it off is anything beyond cosmetic damage.
That is, assuming fusion of your fucking soul is a reversible process.
And I was under the impression that the dark templars who became stalkers were crippled, like the dragoons.