We have body armor for dogs. It can have it's place, although depending on the circumstances, the exigency, heat, whether real trouble is expected or not, it's difficult to say when it will be worn. Understanding that it is protective, the reality is it doesn't protect the head and most shootings of police dogs are going to aimed toward the head. The dog's I'm personally familiar with, that have died of gunshot, body armor would not have protected them.
I use one on occasion when performing high risk entries and when assisting our DTF hit meth labs. David is right though they offer no protection for the area most are killed at. However I believe a little is better than nothing. I now work in the deep south and they can really tire the k-9 out fast with the high heat and humidity here.