Stucco is known to be a weather resistant building finish, but it is part of a system. In order for the wall to resist water penetration effectively, the system must be properly designed and detailed, then built according to plans.
The main purpose of building paper is to keep water from contacting the substrate and structural support membersvery commonly sheathing like plywood or oriented strand board (OSB and wood or metal studsso that these materials stay dry. Metal can rust and wood can rot. Also, wood is prone to expand and contract with changes in moisture, so its essential to keep sheathing dry to provide the plaster with a sound substrate. Minimizing the changes in moisture minimizes the stresses that might be placed on plaster from behind. In addition to structural considerations, excess moisture within a wall creates a potential for mold or mildew inside buildings.
Building paper prevents moisture-related problems in stucco walls. Several industry documents, such as PCAs Portland Cement Plaster/Stucco Manual, EB049, ACIs Guide to Portland Cement-Based Plaster, and building codes across the country, recommend two layers of paper. During construction, paper can be damaged. Two layers of paper provide greater assurance that water wont get to the sheathing or support members. Paper should be lapped like siding, meaning that upper layers are placed over lower layers. This facilitates drainage toward the outside. Where the edges of paper-backed lath meet, connections should be lath-to-lath and paper-to-paper.
Building paper should comply with the current requirements of UU-B-790a, Federal Specifications for Building Paper, Vegetable Fiber (Kraft, Waterproofed, Water Repellent, and Fire Resistant). This specification differentiates weather resistive Kraft papers by types, grades, and styles. Grade D is a water-vapor permeable paper. Grade D paper with a water resistance of 60 minutes (or more) works well for stucco applications, and is often preferred to Grade D paper having the minimum 10-minute resistance required by UU-B-790a.
This argument is just nonsense that takes the discussion away from the fact that there are now holes in the last line of defense (the paper)
None of which is you point of contention.