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?