ABP conducted some sort of ad hoc survey about this [2011]; they write about the results at the bottom of the FAQ list: https://adblockplus.org/acceptable-ads under the question "Do Adblock Plus users really want this feature?"

Supposedly 25% of ABP users were found to be strictly against all ads; 75% are okay with some ads to support websites.

Therefore, arguably, the whitelist feature serves the users who are in this 75% who don't want a blocker to block everything, making the browser extension useful/acceptable to more people.