At what point do you set up for mono?

If you have a decent stereo rig, and a spare (but lesser quality) table/arm, does it make sense to put a mono cartridge on it, or put the $ into an upgrade on the main rig? I guess diminishing returns sets in for each individual at many price points, so perhaps this is unanswerable.

Perhaps my question becomes: does the benefit of a mono cartridge reveal itself in an otherwise compromised chain, or is the recording likely to sound better on a better overall table/arm but with a more modern/stereo-optimized cartridge/stylus?
It depends on the size of your mono collection. I ran a seperate arm wand and mono cartridge and there was a noticeable sound improvement, but in the end, my limited mono collection did not warrant the $ outlay.
If you are going to go mono, then go all the way. Start with a mono source and follow through to a single loudspeaker. The results might surprise you.
Listening to mono via 2 speakers brings substantially better listening experience vs. one speaker.
Dedicated mono cartridge and mono amp with dual speaker terminals will bring best results.
I've recently experimented with Yamaha vintage mono 75...80w powermixer/2xInfinity SM-102/sony 5520 TT with ortofon 1m cart. It gave me an idea that indeed large collection of vintage mono records will sound substantially better with dedicated mono system.