+1, not to mention the very real threat of physical harm from spalling.
I'd never use metal or ceramic plates without a spall guard/liner.
+2
There were some plates over on arfcom that were "found in a warehouse" (a little too fishy for me), and some guys were buying them up left and right. One guy did a test about spalling or splatter and it was pretty evident that they would stop the round but you likely have shrapnel in your face, arms, legs, hands, and whatever else. Then a military guy posted some pics of his buddy with splatter stuck in the top of his mouth, after it had went through the bottom of his mouth and tongue. The guy lived but I bet that hurt like hell.