I did a lot of research on this, but I am no expert. But here is what I learned.

Old bathrooms were built for a lot of ventilation (bad windows etc) and few short showers (small hot water heaters). They allow water through the grout, but most of it drains downward and gets back into the tub or drainpan. Some vapor gets into the stud cavities, but the theory was a little bit would dry. So the first factors to consider - are you in a desert where everything dries? Is this a low use guest shower that will likely dry?

The first improvement to this system was Durock instead of drywall under the tile. Durock is much more water resistant, gets more of the water back into the drain pan, and generally helps a lot. My contractor just uses tile over durock, and guarantees for ten years. I don't know whether to believe him either, but he says Kerdi is not yet proven.

That said, I think there are certain benches and windowsills and such that need extra waterproofing help.

The hidden issue with waterproofing is that if you do get water in the stud cavities somehow, it might be trapped.

You want 6-mil poly or tar paper behind the tile backer board with the bottom of the poly/paper lapped over the tub's flange.

OR

Install your cement board with a topical membrane; RedGard, Hydroban, Kerdi.

You are correct. Tile, grout, cement backer board, none of them are waterproof. They are all inorganic, so water does not damage them. But water can get through the grout, through the cement board, and into your house's framing bays. Thus the need for a membrane, either behind of in front of the tile backer board.

You could split the labor. Have them install the cement backer board and thinset and mesh tape the seams and take the next day off. On their day off, you can buy a gallon of Hydroban or RedGard and "paint" it on the cement board yourself. Two coats. Easy to do. Plenty of youtube videos for reference.

Then they come back and tile.

Are you making too much out of this? No. You want a membrane.

Recently finished a bath. Plasterer hung blue board and cement board. I taped and sealed the tub area using Red Gard.
The tile setter was a little surprised. Most people don't use it because the clients are unaware that cement board is water resistant, not water proof. I would also check with the manufacturer (it is on the container) as to whether thin set or mastic is to be used when setting tiles.Use a good mastic if in fact mastic is called for.
I would also use it on the floor. My fifties ranch had bathrooms stacked and the ceiling on the lower bath was crumbling because of water constantly saturating the floor.

I don't recommend mastic in a tub surround or in a shower. Mastic is water soluble.

Some manufacturers say their mastic is suitable for tub surround and shower walls but prohibit in on a shower floor.

Still...I don't recommend mastic in anywhere in any shower.

I think I will go with the suggestion to add the Redguard myself. Don't know why I didn't think of that.
The tile person I am leaning toward is very interested in how it actually works. I told him about the suggestion from here and he wants to be here when I put it on. Nothing like teaching the expert I guess.
Thanks so much for all of the advice.

While the tile may be water proof (as in stops all water movement and is not damaged) the grout joints between the tiles will wick moisture while not being damaged

Cement board is 'waterproof' only to the extent (like grout) it is not damaged by water.
It still wicks water very nicely, and starts to rot the faces of the studs it is attached to.

You still need a water barrier.

I prefer membranes over ANY field applied barrier.
you are at the mercy of the person painting on a field barrier to leave a correct thickness film and not have any 'skips' or thin spots.

You want a moisture barrier, NOT just a vapor barrier.
It is capillary action between objects, not water vapor that is the problem.

Moisture barriers are ALSO usually vapor barriers, but not the other way around.

It was not as important when a 1+ inch thick mud bed was used.

The moisture that penetrated the grout did a lot of spreading in the thick mud bed.

I just bought my tile guys a gallon of redguard and insisted they use it. Box checked.

Redgard is fine....Hydroban is better, IMHO. To the poster that said Kerdi is "Unproven," BS! Kerdi has been around here in the US for over 20 years and IS "Proven." Properly installed, it will last a lifetime.

You all are more forgiving than me.

If a "professional" comes in and I know more than he/she does, they probably are not getting the job.

How can a tile man never have heard of Redgard or Hydroban?

barbcollins, that's exactly how I had been thinking but now I'm coming to realize that using a membrane must not have caught on around here. When only 2 out of 7 use or have heard of it should tell you something. All of these tile people come very highly recommended from several sources. Amazing isn't it and a great part of my frustration.

Curious... where is "around here".

And to be fair, I have never hired a tile man myself. I do all my own work learning from the experts here on GW.

It does worry me when we buy our next house, if there is a tile shower already, there will be no way for me to know if it was done right.

I am in Southern Indiana. We are about 1 hour from Louisville, KY and that's were most of the good tile people are. Unfortunately if they have to cross the bridge into Indiana and drive out to the sticks, they like to add 3K to the price.
We are also considering doing it ourselves, but we've never done tile before(plenty of everything else) and knowing the way we work, it would probably take us 2 years to get it done.

Oh you can do it! :) If I can do it ... you can do it :)