No particular benefit of Prime solidifies my decision to subscribe but collectively a handful do.

I would imagine most long term members don't take advantage of all the Prime benefits but rather a select few that align with their habits.

The problem Amazon has to juggle is profitablity with members. I would guess if you used all of the benefits then Amazon might lose money on you. So Amazon has to price Prime such that it is appealing to as wide enough audience as possible to offset the loss incurred from people who maximize their benefit and at the same time offer benefits that are niche enough that not all members utilize them while at the same time perceived at potentially beneficial to all members.