OK I'm back after bailing out the swamped crisper, grrrr.
As I was saying, AUD recording of bands playing through a PA is a strange animal. If you were recording a band plaing without any PA you would probably want to place your mics much closer to the band, even on the stage itself. You would then find yourself in a much more typical recording situation in the eyes of the Stereo Zoom, most books on recording, and the way engineers used to record orchestras and full bands in studios and somtimes still do if they're only using a few mics verses multi-tracking. Because the microphones would then be much closer to the band, you'd probably want a wider Stereo Recording Angle and you'd find that following the Stereo Zoom charts would give you a microphone setup that was closer to standard configurations like ORTF and its cousins.
When the same band plays though a PA system, you need to consider the PA as a contributor of sound. Depending on the size of the venue and PA, you may have most of the sound coming off the instuments on stage and just a few things in the PA, or you may have all the audible sound coming from the PA speakers. PA speakers are usually quite directional in how they project sound out to the audience. Partly because of that diretionality, partly because there are often big subwoffers right under the stage, and partly becasue the sound guy is probably adjusting the sound from a position farther back in the room, the sound may be boomy, bass heavy and not so clear in the area just in front of the stage. Because of those things you've undoubtably noticed that the sound is often better if you think of the PA like a giant stereo and place yourself at the apex of that imaginary isocolese triangle. You will then be much farther from the stage than you would have been if the band wasn't using a PA and the Stereo Recoridng Angle from you new vantage point would need to be much narrower, everything else being equal. And so the Stereo Zoom suggests wider spacings and wider angles between your microphones to compensate.
But is everything else really equal? The Stereo Zoom is really about optimizing only one aspects of recording, namely getting the stereo-ness accurate. There are other things to consider that are often more important such as the frequency balance at the mic position, clarity, the direct-to-reverberant ratio, the off-axis performance of your microphones, blathering drunks, the balcony overhang, the noisy bar on your left, the raucous mosh pit, the sqeaky chair where the guy that snores always sits, the HVAC exhaust that blows on your mics, etc.
Retorical quetion- What does an accurate distribution of sound souces across the playback stage mean when 80% or more of the sound you're recording is from a mono PA?
The Stereo Zoom is a great tool, the trick is to understand the ideas behind it so you can decide when and how it is best applied, also realizing that there might be an better tool to do the job in some cases.
Hope that helps.