There's a legal problem that arises too - if you point a gun at someone and pull the trigger, you are considered to have used deadly force, regardless of whether or not you have blanks in the gun.
If you have a blank, then you face serious questions about whether or how justified the shooting actually was, and whether you truly thought that deadly force was necessary.
If you use deadly force - including drawing and firing a gun with a blank - without being in reasonable fear of imminent great bodily harm or death, then you may wind up subject to prosecution. And if you're in fear of GBH or death, you don't want a blank, do you?
Further, an acquaintence of mine was menaced by four young toughs who jumped out of a car and started towards her after a couple of crude and vulgar drive-bys. She was pissed at herself for leaving her spare mag at home, leaving her with only 12 shots. A five shot revolver against four young, healthy attackers? Good luck. Lucky for her all it took was her reaching in to her fanny pack holster for them to cut and run. If they were drunk or on drugs, she might have had to go for slide lock.
Only 2% of defensive firearm uses actually involve the discharge of the firearm, I've read. The mention, the display, or the aiming of a firearm is most often all it takes to dissuade an attacker. If one finds oneself in that deadly 2%, I expect that one would desparately wish they hadn't deliberately crippled their defensive tool.
Not a blacksmith could be found in the whole land of Israel, because the Philistines had said, "Otherwise the Hebrews will make swords or spears!"
1 Samuel 13:19