They work best when the car isn't moving. As far as the strange player goes, I'll keep the ones I have. Not too sure what they would do to '78s from the 20s and what those '78s would do to the cartridge.
If one is sitting in the parking lot, waiting, then I suppose being able to put on a few discs to pass the time wouldn't be so bad. But I would think the easier thing to do would simply be to turn the radio on and listen to Max Ferguson.
When I was much younger, I had a record player that did all speeds from 16-2/3 up to 78. My mum would buy me kids records (I still have a Lone Ranger 45 from then) from the corner store, that also served as post office. We also bought our needles there. Interestingly, styluses at that time - pre-"Hifi" - were steel. You'd loosen a set screw at the end of the tone arm, insert a new needle, and tighten the set screw. The needles were essentially little straight pins. You could get a couple of dozen plays out of them, by which point they would have lost
their point...which is why they were sold in, as I recall, little envelopes of 10 for a quarter. I'm assuming the cartridges were crystal.