When a user sees a banner on a site, they see it as exactly that - a banner. They don't see it as distinct elements on the page, like they would other items. Because the user doesn't see the banner as a group of distinct entities, they are just as likely to try to click on whitespace as they are text or an image. Because of this, banners should always be treated as a single entity when it comes to what they link to.
Also remember that many users at this point have "banner-blindness". If they see something that looks like a banner, they automatically ignore it.
If they really want that area to represent several paths that a user could take, then it should be changed to represent that - make it not look like a banner, and users won't treat it as a banner. They will be more likely to know that there are different things to click on, and they will also be more likely to do so, since the content won't be subconsciously ignored.