The following is the excerpt from maven documentation on resources element of build settings from pom.xml:
"Another feature of build elements is specifying where resources exist within your project. Resources are not (usually) code. They are not compiled, but are items meant to be bundled within your project or used for various other reasons, such as code generation."
The key point here is that directories mark as resourcea are NOT compiled. If I define such resource in my pom.xml IDEA will add this directory as a source dir upon maven project synchronisation. Suppose I have a java class in one of the 'resource' directories which is intentionally not compilable (this can happen for GWT 'translatable' classes for instance). I will not be able to compile such project from IDEA (using 'rebuild project' menu item) while the project will be successfully compiled when using maven. Moreover, when packaging such project using maven the package will contain java source code from resource directories while if doing the same via IDEA the package will contain compiled java classes from the resource directories.
Looks to me like a concept mismatch. IDEA treats such resource- directories like a source directories while it is more like classpath dirs (but not 100% since the content of such directories must be copied to the output dir as it is in the resource dir).