I've been to Kona Cafe many time, both for breakfast and lunch. I've always had a PS. There have been times, where there are only a handful of other people in the restaurant and they will seat people right away. I've also been there (this usually happens at dinner) that even though the restaurant doesn't look all that full, they refuse walk-ups or will give a time that's 30 - 45 minutes away. My guess is that there are a lot of PS's that are in their computer and can't give those tables away to walk-ups. This has also happened at breakfast, either turning people away, or long waits. I always have the mantra, if you know when/where you want to eat at WDW, make a PS. To the poster who's planning on eating at Kona on July 3rd, I'd recommend a PS. That's a holiday weekend, and unless you plan on getting to Kona at 7:30, a PS would make your wait effortless.
Edited to add: I usually go to WDW either during the 1st 2 weeks of July or Christmas break, nowadays.