I have done quite a bit of research and thinking on this subject. Before you can dismiss mono cartridges as a class, I think you have to consider how the particular mono cartridge(s) you are evaluating was constructed. Only a few were constructed so as to be unable to respond to vertical motion of the cantilever/stylus. One example of such are the Miyajima mono cartridges, which were designed to be mono in the first place. (TSD and Ortofon may also make "true" mono cartridges.) Almost all other mono cartridges are stereo cartridges that have either been modified internally such that the capacity to respond to vertical displacement is reduced (and then the two channels are bridged at the output) or are merely stereo cartridges where the two channels are bridged at the output. (Art Dudley reviewed 3 mono cartridges in one article wherein he tested each for its capacity to reject noise due to vertical motion of the stylus; the 3 were very different from one another in this capacity.) Obviously these 3 basic approaches can make quite a difference in the degree to which a given mono cartridge can be superior to using the mono switch on one's preamplifier. This is why, if I buy any mono cartridge, it will probably be a Miyajima, but then I would still use my mono switch. Unfortunately, the preamp driving one of my two systems lacks any mono switch (Atma-sphere MP1). I am thinking of replacing the function of its "phase" switch with a "mono" function. Miyajima or no Miyajima, I would use BOTH the mono switch and a mono cartridge, optimally. (Have you tried that, Henry?)
But Henry, you did give the impression that you were comparing the dirt cheap AT mono to some very expensive stereo cartridges, although I don't believe you named them specifically. Meantime, I am quite pleased with the effect of using the mono switch that is available on my second system; it's such a big improvement over playing mono LPs in stereo that I don't play mono LPs on my main (no mono) system any longer.
But Henry, you did give the impression that you were comparing the dirt cheap AT mono to some very expensive stereo cartridges, although I don't believe you named them specifically. Meantime, I am quite pleased with the effect of using the mono switch that is available on my second system; it's such a big improvement over playing mono LPs in stereo that I don't play mono LPs on my main (no mono) system any longer.