As far as slinging goes, if you use a big enough rock, the helmet may save his skull but the impact will break his neck.

In general it does seem that armor does work... that is the whole point of going through all that trouble of expense and encumberance... but no armor is perfect. Yes, a good helmet does shut down most instakill sling shots, but a helmet can be worn down, one can still get a concussion through a helmt, it is true a neck can be broken, an opponent can still be knocked down (or off a horse)... all dependent on the nature of the particular helmet of course. But here's the other thing... helmets only protect so much... what about the face? The neck? and oh I don't know... the rest of the body and limbs (especially joints). Also for most of history the majority of soldiers have no armor at all including helmets... and many helmets through history are not of the utmost quality... brass and bronze often bending in anyway (depending on thickness) and iron either having the same problem or flat out cracking and breaking after a sling hit... better suited to deflecting the points of spears and arrows... and better at resisting the lesser impact of hand thrown stones.

The example Fundibularius quotes (from Caesar's Gallic campaign) has a Roman commander hit in the mouth by a slingstone. I think Paullus was hit in the head by a slingstone at Cannae; at Thermopylai, Antiochos III was hit in the mouth by a stone-- hand or sling ? (and lost teeth-- he had already lost some in a cavalry battle in Baktria). Literary sources, but perhaps show that a long-range, depleted-energy stone hit to the head is survivable.