It truly depends on your personality and values. It is probably breat to hear different point of views from here. But it is really you.
Personally, I can seldom listen to a mono recording… I get bored almost immediately. I loved a Fats Waller album but am so disappointed it is not in stereo. It just falls flat. Another personal thing… I want the best possible sound at all times, so I would never own two cartridges. Never.
Alternatively I have read about folks with nearly a dozen cartridges with head shells… pairing the perfect cartridge with a particular album… I am left scratching my head.
On the other hand, I shoot a Leica M10 and have been very tempted to buy a Leica M10 monochrome (only shoots monochrome… higher resolution and more gray shades).
So, it comes down to you. Given your question I like the idea of splitting the investment in proportion to how much you listen to stereo versus mono.
Also, from my experience if I have a $10K budget… definitely worth spending it. My cartridge is around $8.5K, definitely well worth it.