I did a little checking in some of the tiling books at the local Big
Box store and many of them advocate a membrane be put down on the
bathroom floor to help prevent water getting to the subfloor. Is this
really necessary or is it overkill?

Based on some of the floors I've seen, it's not overkill.

A client called just today to have six one-inch tiles put back down on
her shower floor. That's normally a quick fix--chisel off most of the
old thinset, then put it back down with either thinset or mastic.

I removed the old tiles, which were lying loose in place, and found a
hole full of sand. The old concrete has decayed down at least two
inches. Of course, they don't have the money to pay for the correct fix
(put in a new base, pan, etc.), so I'll get out the junk, fill the void
with concrete, and put the loose tiles back with thinset.

This is a good reason to use a membrane.

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