It would give intercepting fighters the biggest surprise since the Bf109s attacked Defiants from the upper rear thinking they were Hurricanes!
Well, it wasn't thought just as surprise. It was an idea for the situation of German fighters being hopelessly outnumbered (as EKB stated, you remember). The propeller blades were made from armour steel, being left from a stock for German battleships which remained unbuilt, so in fact they were "propeller plates". The tactical calculation was the bullets of the machine gun were deflected in every direction by the propeller plates, so even a big number of pursuing Allied fighters could be hit at once by using only one aircraft with only one rearward-pointing machine gun. I am not quite sure what happened to the project, probably the war was over before it could be realized.
Also, I like pictures of the Defiant showing the machine guns in a forward-pointing position, it makes me try to imagine how the mechanism of automatic fire control might work, although I confess I am unable to...
(Would like to post a picture, the program doesn't let me, don't know what happens).