Since SlickEdit's tagging for C++ does not parse header files included by the current source file, there's no pointer in doing preprocessing assignments within the current source file.
The advantage to what SlickEdit currently does is that users don't have to specify the precise include file search path for every project and it's much faster. You just have to add files to your project if you want them tagged. Users typically don't want to take the time to define their includes for their projects.
In the future, we may offer multi modes of operation.