This is all getting a bit "handbags at dawn isnt it"
Can I just clarify. there seems to be a tanking is not essential opinion on one side and an it is essential on the other. Is that right.
Deano your own experiment showed that water get through the grout so I don't understand how you can accept the BAL reps opinion that water behind tiles is ridiculous. Grout is not impervious to the passage of water. Showers get wet. Ergo water gets behind tiles in showers. The amount is the key. Is it enough to do damage and how does that damage occur.
The need for tanking would therefore depend more in then on the implication of that water. If it is getting through to a highly absorbent and substrate which will suffer deleterious effect over time e.g. plasterboard, Ply or heaven forbid MDF then I would say tanking is essential. If it is a brick or block wall that has been sand
cement rendered, or a
cement board lined cubicle then is it an optional safeguard.