There are two primary reasons steel stud parapets are advised against. The increased possibility of water penetration and resulting corrosion is one issue. A parapet is externally exposed on three sides making it one of the most vulnerable areas of a building. Furthermore, it is insulated differently than the wall below and can potentially be subjected to more thermal-related issues than other parts of a wall. Water penetration is also more likely due to rain or condensation. This water penetration could lead to corrosion and other future problems.
The second issue deals with the differential movement of the brick and steel stud. The stud can expand at a much higher rate than the brick. This could effect the coping used on the top of the wall and surrounding mortar joints.
Obviously, these issues can be dealt with in design and construction, but BIA has seen some problems in the past with this type of assembly.