A few weeks ago, the derailer on my touring bike caught on the chain and ripped itself off the frame. (See the link below for more info on the incident.) The spoke protector was cracked in half by this, which says to me that it did its job: stopping the derailer and chain from damaging the spokes. (The wheel was out of alignment and needed truing, but it still rotated.)
In the future, these "dork discs" or "pie plates" (as some like to call them) are going on all my bikes. My question: Aside from looks and the weight, is there any downside to having one of these on a bike?