I think it was RMS who argued that conditional compilation should be considered harmful in general. Code that you put in an inactive #if(def) block will not be maintained, and is basically guaranteed to rot. If it's needed in the future, it'll likely have to be rewritten from scratch.

According to this stance, any code that's suppressed by the C preprocessor should either be written in an if {} statement so that it will at least continue to compile as the surrounding code changes, or be replaced with comments describing what it does (or did), if it's important enough to keep track of.

Can't really think of many good counterarguments to this. Machine dependence might be one, but then you could argue that the preprocessor is being used to cover up for an inadequate HAL.