The helmets were not gonna save their lives. They were to strike fear into the enemy.

https://www.kalkriese-varusschlacht.de/en/the-varus-battle/the-romans/the-roman-army/equipment-and-clothing/

"Protection of the head was provided by iron and bronze helmets which were called »galea« and »cassis«. To deflect hits and to attach the crest, which was mostly dyed red or black, they sported studs or forked crest holders. Neck guards and cheek pieces attached with hinges offered additional protection."

Um...no. The Romans struck fear into their enemies by being the best military power the ancient world ever saw. The helmet was to protect the head.

Given the commonness of slings as a weapon of war, I doubt that. A helmet was the best protection they could use against thousands of flying rocks.