Just curious how that might happen? The adjustment screw creates a stop for the derailler, keeping it from moving too far to hit the spokes or put the chain behind the freewheel or stick it between the freewheel and the frame. The stiff springs on the screws keep them from moving off the set adjustment. In over 30 years I don't think I've had a derailler go out of adjustment without my knowledge! Granted, if a bike is damaged those alignments will change. But even if you lose all tension on the cable and it shifts to the highest gear, the derailler is still contained by those limit screws.

I imagine the dork discs were added (actually, they had em even in the fifties, but they were pretty small and all metal) because the dork would try to "fix" their bike by twidling with the screws without knowing their purpose or setup. In that case, the disc would prevent damage when it went into the spokes. But it was the first thing removed by any "serious" cyclist since their invention.