Technically, WD40 has nothing to make it rinsable (as a dedicated tar remover would). This means that you need to take special effort to then clean the solvents away - a rinse with water is not enough. Worse, WD40 is specifically designed with additives which are highly water repellent, so not only do you have solvents which have no easy mechanism for removal, you have additives which are notably difficult to remove. What this all adds up to is something which has a high potential for leaving misc residues if you don't give a really deep clean after.
That said, this is one of these areas where detailers, who have limited experience with real tar removers, are willing to, half blindly, state that the above technical reasons are rubbish.