Cement backer board over drywall

[Greg Clark]

Just pulled off some paneling on a rehab job and behind it was a thin layer of plaster. Then, 1/2 inch Cement backer board. THEN drywall.... 

So from outside in, it's vinyl siding, weatherproof wrap, sheathing, studs, drywall, backer board plaster, paneling. 

Why would someone do that? Was it common to put Cement or plaster over the top of drywall in the early 50's?

And what do I do about it? I'm considering either removing it down to the studs and putting up drywall or just adding a layer of 3/8 drywall over it to smooth it out and calling it good.