I'd use a waterproof board anywhere thats likely to get wet (or even condensation) regardless of what's on top.  

Wedi board looks like blue polyeurathane insulation foam with a thin layer of plaster/mesh on both sides. It's very light and very easy to cut - eg with a stiff knife or hand saw. It's harder to fix into (eg towel rails, soap dishes, shower head brackets). I built a wash stand from it and added wooden blocks where I knew I was going to fix into.  

Hardi backer board is much harder, tougher and heavier than regular plasterboard. Our builder used this on the whole bathroom under 10mm stone tiles and it's practically bomb proof. Hard to cut - our carpenter used about 20 jigsaw blades to do the job.