Yesterday, my wife called twice for PS, with about 15-20 minutes between calls (as we discussed our choices), and some of the spaces available on the 1st call were gone by the 2nd call. She thinks WDW does this to hype their restaurants, but it means essentially you can't plan attending that favored choice without that PS.
Some other threads on this board had folks saying they never make PS's and they just show up at the restaurant of their choice. If something is available then they wait 15-20 minutes, if not, they just go somewhere else or go back to their hotel for dinner.
All this concern for landing those PSs is necessary to not disappoint the kids, but takes a lot of fun out of the vacation. I mean having to program where you eat breatfast, lunch or dinner on a vacation?