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Interesting - I thought the pin on the plate interfaces with the spindle, preventing - perhaps a bit optimistically - the arm rotating around the spindle over the splines, which is where I'd imagine the catastrophic damage to occur. Having it rocking around loose on the spindle isn't going to do the arm any favours, either, I know.
Howard-
The pin's about 0.5mm ΓΈ - not going to stop anything given the forces on the crank spindle. I always thought it was more to let you know you've got the cranks in the right position since if you don't, the pin won't engage into the hole in the spindle and the plate will be in the way when you try and insert the clamp bolts.
I'm surprised the plate serves any purpose other than to stop you fitting the cranks in the wrong position.
rhowe
Nah, the "safety catch"(it's just called 'Plate' in the instructions) is just there to add friction to the clamp bolts, like a Nyloc, to reduce their tendency to loosen in service. If the clamp bolts have too little tension, the splined interface oscillates under load and since the axle is steel and the crank aluminium, the crank tends to get wrecked first.