Only if they are likely to fall into the wrong hands. (I.e., anyone but their operators.)
There have been coaxial helicopters as well as various twin-rotor types over the years. Some of them had very closely-located intermeshing rotors, others had a rotor on each end of one on each side.
I'm
guessing that the reason we don't have many today is more than just economics... But it is just a guess.
Like I said earlier, the airship thing is a great idea. A neutral-buoyancy craft doesn't have to fight gravity. But to get to neutral buoyancy, most craft will have to be large enough that severe weather is a severe problem.
References for my earlier statement about the airships lost to weather:
https://en.wikipedia.org/wiki/USS_Akron
https://en.wikipedia.org/wiki/USS_Macon_(ZRS-5)
https://en.wikipedia.org/wiki/USS_Shenandoah_(ZR-1)
It appears that the Navy used a grand total of five rigid airships. One was destroyed in testing aggressive maneuvers before delivery, and one actually survived until its decommissioning. The other three were lost above. Not a very good record...
-soD