The phase shift is really useful if you’re a DJ playing out on a loud system with a good chance of mechanical feedback. i.e. there’s a chance your cartridge wi

The phase shift is really useful if you’re a DJ playing out on a loud system with a good chance of mechanical feedback. i.e. there’s a chance your cartridge will pick up sound from the speakers. The feedback will be picked up exactly the same on both the L and R channels so if you combine them to make mono the inverted channel will cancel out the feedback on the non-inverted almost perfectly. The stereo image isn’t usually useful in a venue setting anyway so there’s not much downside.