With today's much improved derailers the chain will not into the spokes if it properly adjusted. But what about if you crash & thrash the derailer itself. Then the disk migh be needed to prevent the chain from shifting into the spokes, right?
The only reason I ask is because the disk on my bike was recently replaced. It crasked & broke just after 5 years of riding my bike. Didn't crash or anything it just broke. I had it replaced after my LBS mechanic told me it is more for in case you crash which can send the chain into the spokes.