In the case of the 21" color tubes I think the safety factor comes from the bonding, if the tube gets broken it will hold the glass shards together the same way it does in a car.
This is one reason I have a problem with the practice of reattaching the safety glass just around the edges of the tube, if it gets broken you are actually going to have twice as much glass flying around!
That said, I have a NOS rebuilt tube that I cracked the safety glass on and will have to do surgery on at some point.
