Defining LIBFOO_DLL_EXPORTED in public header files has no added value from the user's perspective: mentioning a function in the public header files implies, that the function is visible. As suggested, there is another way to achieve the same result, without touching the header files.
One advantage I see for decorating declarations is easier compatibility with compilers like MSVC that require exported variables to be decorated with dllimport. From a C++ point of view, I'd also find easier to decorate the class declaration once, instead of having to decorate all members in the source code.
Michael.