In a typical bathroom floor tile installation you have the following
sequence:

Plywood subfloor, Thin-Set, Backerboard, Thin-Set, Tile/Grout.

If the grout is not perfectly sealed and the bathroom is heavily used
(with a lot of water on the floor), where does the water stop? I know
Backerboard is water-damage-resistant, but I hear it can wick and
absorb water. What are we relying on to keep the water from soaking
the plywood? Is it just the thin-set?