I've done one shower floor, and several kitchen counter tops, in ceramic tile. Never used any bonding agent in the thinset, as the stuff is designed by itself to "grab" the tiles and tightly hold them.
That being said, CallMe, are you absolutely sure there's no water leakage into the shower walls? I've come across lots of tile showers in older homes where water has gotten into the walls. You might want to stab a moisture meter through the back of the walls sheetrock (in adjacent rooms) to make sure there are no surprises. Had one last summer for a Department of Human Services client (house built in '60s) that was leaking so badly that the sill plates and bottoms of all of adjoining studs were "mush"--meaning thoroughly rotted. Had to take out 2' of every stud, sister in replacements, along with new treated sill plates.