Yeah, I'm not convinced that FP+ usage on Haunted Mansion has reached any point where it would negatively impact standby waits. Sometimes things are just what they seem--lines are longer because more people are visiting.
IMO, implementing FP+ at lesser attractions is about 50% PR and 50% geared specifically toward the highest volume days of the year. There ARE actually times when rides like Haunted Mansion, POTC and others have long waits. We visited on President's Day a couple years ago and waited nearly 30 minutes for Nemo and about 15 min for Figment. To some people, riding Haunted Mansion on NYE with no wait is just as valuable as avoiding the 40 minute Space Mountain standby on a much slower day.
As for the PR aspect...there's a lot to be said for guaranteeing zero wait on a few attractions before the guest even buys that $100 ticket.
