My friend and near-neighbor, Fred Rafael Rednor, wrote:
Harvey, Here's my guess. Cinelli didn't use the 7mm opening to be different. In fact, TTT used this size as well. I believe they felt that you couldn't generate enough torque with a 6mm hex key to properly tighten the quill's bolt. Remember, this was not so long after the time when all stems used an ordinary hex headed bolt head. (10mm to 12mm if I recall correctly.) <snip> +++++++++++++++++++++++++++++++
Well, Fred, you raise an interesting question when you talk about "generating enough torque with a 7 mm hex key to properly tighten the quill's bolt."
I've always felt that stems should be just tight enough to not turn in the steerer under emergency conditions -- and no tighter. I've always tried to set things up so the stem would be loose enough to give while crashing, since that might lessen injury.
Am I overanalyzing the situation? Does the rest of the world just clamp on down till it is real tight (the way I do at the other end of the stem, so the bars don't rotate when I stand and apply the Mighty Thrust of the 61 year old body)?
Your mileage may vary.