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?

You forgot the sealer.

You're relying on a properly-installed and well-maintained floor.
Properly-installed means all the layers were put down as specified by
the manufacturer, without gaps or cracks. Well-maintained means
applying sealer every couple of years (more often if heavily used),
repairing any chips or cracks, and mopping up spills as soon as they
occur.

None of the layers you mention is actually waterproof except the tile,
and that's only waterproof if it's glazed. (Everyone feel free to point
out some type of tile that I don't know about.) The sealer fills small
cracks, including the pores in the grout, but it wears out.

I can tell you from experience that water gets through or around tile.
I recently relaid a bunch that came loose from plywood because of a
leaky roof. (The client didn't want if fixed right, because he plans to
tear the building down in a year or two.)

--
Steve Bell
New Life Home Improvement
Arlington, TX USA