dont think it is a necessity but like you said as far as keeping an eye for the album...when you see that album in stores that you saw on a flyer,you are going to recognize it and maybe be more intrested than maybe you had. without the cover on a flyer i dont think it will stand out as much as you want it to,unless you plan on having another image
not to mention a cover can tell alot about the artist and what he is trying to do on an album,so if your cover catches their eye it will just get that much more intrest from them