Just because they may be enabling the Haunted Mansion for Fastpass doesn't necessarily mean they plan on installing Fastpass (in the long term). In this age of Fastpass, they need to be sure ALL lines can accommodate Fastpass, so that they don't have to rig one after the fact if they (for some reason) need to implement one. You never know when a refurb or a new Haunted Mansion movie (or a holiday) might boost an attraction attendance. Maybe the next phase of fastpass includes quickly mobile fastpass machines so that they COULD set them up quickly during peak times...
...because a body in line doesn't spend money and isn't as happy as a body wondering the park.