Safe ??? i don't think the driver uses the car with it hanging, i think its a great idea to combat thoughtless sods that just open doors with no regard for the car next to it.
I was where you are now once, bought a Lincoln half decent machine to relieve my urge and after 12 months of rent on a bottle of Argon with only 20% used i knew it was an urge too far :(...machine sold and bottle returned and battled on with MIG.