I think they are important if you really want to eat a particular place, or are trying to time things well. You know? Like, if you want to make the nightly show which is at a certain time, and know the restaurants will be crowded, its good to have a PS so you can get fed and out in time.
we usually plan our days pretty detailed... so we don't miss things like parades and shows and eat at the places we want to eat at. So it isnt hard for us to figure out where we'll be at a certain time and what places we've always wanted to eat at there.
You can always cancel them without penalty, so I think its worth it. You can do all this ahead of time before you go.
THis time, we only made PS for the princess breakfast (obviously), Coral Reef, and Ohanas. We will just punt on other things, and probably eat cheap. But for nice meals,I hate waiting forever. =)
Good luck!