Usefulness is complex: "top 50" was a hack that made sense in a small and novel app world; it makes little sense now that so many tastes are represented in the iOS user base. Pay-per-install is one way to shake more material into view and see if it sticks.

Apple makes money (and users benefit) by putting new material in front of eyeballs. They're going to be very careful about how they tweak the rankings.