We tend to PS for a few meals each trip. Especially if it is a busy time of year, or there is a particular event or timeframe I specifically want.
For example:
We usually like to do CG once each trip, and like to time it for fireworks viewing. For this I ALWAYS do a PS.
With a party of 5 for specific meals we really wanted to have together last summer, we did a PS.
This coming summer we will be at VWL for 7/3 & 7/4, and we have a PS for Ohana for Thursday, and one at AP on Friday.
They may not always be necessary, but better to have it and not need it than the other way around. You can always change your mind. If you simply don't show, someone else will be seated faster. The whole point of the PS is to have the next available table. These are not "reservations" in the true sense of the word, so nobody is holding a table for you. It certainly is more courteous to call and cancel if you can.
OTOH, for our 7/4 ressie, I had to give a CC that would be billed $10 per person if I don't show and don't cancel 48 hours prior. So for a holiday, you definitely should obtain them.