Mono Cartridge, is it worth the investment?

no its not

I have a table with two arms just for that purpose. One has my mono cartridge. Although I don't have a lot of mon records, the mono records definitely sound better with the mono cartridge than a stereo cartridge. If I had to swap headshells or arm wands to play mono reocrds, I wouldn't bother. Two arms is the way to go if you want to get a mono cartridge. It will cost you some bucks, but hey, what else is new with this hobby!

I am using the Graham phantom with one arm stereo Jan allaerts Finish and another with Lyra Helikon Mono cartridge. Swapping arms takes me 3 minutes.
Sound benefit with the mono cartridge is huge with records pressed before1960. On recent reissues of old mono records is less audible. So if you have original records mono they deserve a good mono cartridge, you will be surprised how good they sound.

Slowhand, if you have a VPI tonearm system it is even easier to swap armwands,all the settings are done only one time, so it takes 30 seconds to swap arms !

Slowhand - I appreciate your developing interest...I'm noticing the same thing as I get back into Lps. Those old 50's mono records sometimes sound really nice. But what a pain to collect! Finding them in decent condition, cleaning them, etc., and the results are all over the place. But some of the old Ansermet, Monteux, Toscanini...fun stuff! A mono cartridge is on my list, but I don't see how I can do it simply without a 2nd turntable, for which I don't have room. In support of Jloveys' comments, however, I've certainly heard from the "old hands" that mono cartridges help those old mono records sing!