It can be. I think that this distinction between art and illustration is a rather weak one that was created in order to segregate the insane world of modernism from the practical needs of people to use artistic communication in conjunction with ad campaigns, books, and so on. When someone paints a great painting of a woman washing her hair with Head and Shoulders he might be doing it for a commercial purpose, but he's still making art (if he does it right, that is). When someone illustrates a chapter of a book, or makes a poster to promote a movie he is still doing everything a conventional artist does and he is still doing it the same way, he's just doing it for a commercial/product promotional reason rather than some other one, and I don't think that there's anything unworthy about that. The existence of talented "illustrators" in the 20th century in fact has been the carrier of much of the good knowledge and technique from the time before modernism to the present day. That they often had to work for commercial interests like magazines, book publishers, and ad agencies says nothing about the quality of their work (other than that people with genuine expressive needs recognized their value) and it says a lot about the fact that such people were mostly shunned by the rest of the "fine art" world. The "rehabilitation" of Norman Rockwell in recent years has been a good sign in this regard.