Well, to be fair Pete, you're right. I guess that sometimes the un-sparkly humbuckers can use that extra bit of slice, but as soon as an overdrive or distortion pedal is kicked on, I really don't like that extra buzz on top that the bright switch adds. But with single coils, I've never found the bright switch to be beneficial. Sounds like adding a tweeter to the speakers.
Technically, the bright switch allows all the crispy highs to pass thru (or around) the gain pot whenever the pot is anywhere less than 10. For example, if the amp gain is on 3, the crispy highs sound like they're on 10, so it's dramatic. But if the gain is on 8, then the crispy bright-switch highs are on 10, which isn't really that much more than 8 so it's a subtler effect. As you crank the gain, the bright switch's effect diminishes. With the gain on 10, the bright switch does nothing at all, as if it's not even there, no bright boosting whatsoever. With Jerry's typical gain setting of 4, the bright switch is VERY pronounced sounding and to my ears is very UN-Jerry. There's already plenty, gobs of crisp sparkle without it. The "meat" of Jerry's tone lives in the musical treble range, maybe in the 1kHz to 4kHz range. The bright switch boosts stuff well above that, like 7kHz on up past 16kHz. With the buffer, the single coils, and the JBL's, that can be an unpleasantly abrasive and glassy characteristic in a guitar tone that pulls the ear's attention away from where the musical harmonics live.
... and it's just like any other day that's ever been...