All showers need to be water proofed. This includes shower pan, and walls all the way up to shower head height. Cement board alone is not water proofing.

the roofing paper is a curing membrane for the mortar bed, and the fact that it is there does not necessarily mean that no water proofing was installed on top of the mortar bed.

You need to find out what water proofing system was used for shower pan and for the walls before we can comment further.

First off, I need to say that I have a great deal of respect for the majority of the experts that offer advice on this site. However, I feel the need to remind everyone that the modern waterproofing systems (and uncoupling membranes) were not always availible to us. I have been involved in this business for over 20 years and when I was apprentacing, there was no Kerdi or Hydro-Ban or Aqua Defence or RedGuard availible. There wasn't even DenShield. There was drywall or green drywall, plaster, and cement board. Those were pretty much your choices for the walls. And our showers didn't leak. I honestly can not remember every having a customer call us back because their shower was leaking water.

Today we have the option of using waterproofing membranes, and I strongly recommend using them. Today, my own bathroom would be done using Ditra for the main floor and a Kerdi/Hydro-Ban hybrid system for the shower or tub-surround. But many people still don't. A lot of people don't want to pay for it. For some, the extra $2.00/sq. ft. for the Kerdi plus the extra labour to install it over the already moisture resistant cement board seems like expensive redundancy. You can suggest it as strongly as you like, but many customers would rather save the extra $300+ because 20 years ago it wasn't used and the showers still didn't leak.

DIYers only have to pay for the material. If you're paying for both material and labour it gets expensive. Dispite what you might think from reading this forum, as a professional tile contractor I can tell you that when you are dealing with homeowners on a budget, Ditra and Kerdi are a hard sell.

The first place I would look for a leak in this shower is the pan. If the pan is not waterproofed properly from the outside, it needs to have a proper pan liner and weeping system under the concrete base or it will leak. Not probably. It will leak.

Mold was not considered a big deal 20 years ago either. Today we are well aware of the risks of exposure to even small amounts of mold spores. So you are correct that not water proofing shower walls will not lead to leaks, however it can lead to moisture migrating into wall cavities and encouraging mold growth.

This particular shower is a doorless setup and would be relatively complex to water proof if a liner was used.

So basically we agree, except that I will never install a shower without waterproof walls. If the customer doesn't like it, they can find someone else, but I have yet to find one that doesn't want water proofing. A custom walk in shower with the glass and the plumbing, etc. is usually a $7000 or more project, so $300 this way or that is not a big deal.