I personally don't agree that it's a waste of time & money? Okay, Hardiebacker not waterproof but its moisture resistant not like plasterboard which soaks up moisture like a sponge & after time so does the moisture resistant plasterboard. Also, I've stated on more than one occasion I trust Hardiebacker & this is a forum isn't it
Hardibacker soaks up water just like a sponge too.
Only it doesn't break down, but it WILL pass that water right through to the timber floor, studs, joist whatever.
Hardibacker is rubbish for wet areas and has no place in modern bathroom construction (in my opinion).