Early and late in the day I see it all the time. Partially filled boats on POTC, JC and IASW. Some DoomBuggies on HM sent out empty. Empty seats on BTMRR. When I was at DL a few weeks ago we rode Alice in Wonderland and the Tea Cups with
many unused vehicles/cups about an hour after park opening and two hours after Magic Morning started.
Think of it this way. Assume for a moment that DL increased the number of FPs issued.
It is common knowledge that operators run many rides purposely at reduced capacity early and late in the day. Instead of five trains running on BTMRR they run two and then ramp up to five as the lines build.
FP causes people like me to grab FPs in the morning, skip Space Mtn and Splash Mtn until mid-day, and go on rides running at under capacity. When I do this I am not riding those rides when they do get to capacity which frees up a seat for someone else. If not for FP I would be getting in the Space Mtn and Splash Mtn lines first thing and
not going on Alice or Tea Cups. With FP more total riders can go on rides in a given day.