Fact or misinformation?

I haven’t had a mono cartridge since the early seventies and have never had an issue.

All the best.

The big difference between a mono cartridge and a stereo cartridge comes down to noise pickup.  A true mono cartridge has the coils oriented to only pickup info in the lateral direction whereas a stereo cartridge is oriented with the coils at 45° to pick up info in both the lateral and vertical direction.  If converted to mono at the cartridge, the phasing of the stereo coils is such that the lateral info sums and the vertical info is nulled.  this null only becomes perfect if the info in the vertical direction is identical in both channels which never happens in the real world.  If the summation to mono is done by a mono switch this noise cancellation is less. The net result of this is since a mono record has only noise and zero musical content in the vertical direction,  a true mono cartridge can be much quieter in the groove than a summed stereo cartridge.

at the end of the day a cartridge is a compliant lever with a needle and either one or two electro-magnetic coils at the other end (or magnet moving inside a coil).

Mono records only have a single direction of displacement.  Stereo have two.

But either way its a compliant diamond sliding through wiggles. A stereo needle will simply pick up the exact same movement on both channels.

I do not see how this will damage anything.

I admit i've never looked closely at how they align the squiggles

So i find this assertion very unlikely

Sadly, it was the owner of the retail establishment who gave me the no-go request.

One exhibitor confirmed my thoughts as to why some wont. If he puts on a request that isn’t popular, he risks driving out the traffic in the room. I get it, so I make them during lulls in room traffic, or I their LP selection is similar to what I’d like to hear.

I’ve gone to shows over the last 3 decades and that one was a first.

I also got the vibe the dude really didn’t want to take request and just spit out the goofy excuse in hopes I wasn’t informed. Sigh.