My experience does not match up with the info on the Hoffman site at all, namely that high frequency info is compromised using a mono cartridge on mono reissues or mono records cut using a stereo cutter, or that mono cartridges sound inferior to stereo cartridges on post '68 monos.
FWIW, I use two Orfoton MC 20 Supers, one strapped for mono on what is an arguably slightly inferior arm and another on the better arm for stereo. Both on the same table, both into the same phono stage.
In the interest of full disclosure, both Ortofons have been rebuilt (and IMO are significantly better than original), one with a boron cantilever and microridge stylus (that is used for mono but I put about 1000 hours on it playing both stereo and mono records before dedicating it entirely to mono, so am quite familiar with its sound), and the other with a sapphire cantilever and microridge stylus also.
Both cartridges were rebuilt by Andy at phonocartridgeretipping.com but the boron version did need a completely new coil. The result is that it has a slightly lower output than the other MC 20 Super (not really a big deal as my phono stage has infinitely adjustable gain to compensate for this); not sure but it is possible the internal impedance is slightly different as well. Over the years the MC 20 Super specs seemed to vary in the output department, from .2 mV to .25 mV and the internal impedance was also quoted differently at either 3 ohms or 5 ohms.
In any event, probably about the closest you can get to comparing the same cartridge in stereo vs mono mode; it's at least closer to most of the comparisons I've read in the forums which is, as folkfreak suggests, a bit of a problem.
There's no question that, even with a decent strapped to mono cartridge, the presentation is significantly improved on both vintage and modern mono pressings. It might be easy to confuse that presentation with one that is inferior in the high frequencies as (at least IMO) the presentation with a stereo cartridge is thinner, more lightweight and with more sense of "air", resulting in a sense of perhaps more high frequency info being presented.
In my system at least, this is a bit of an aural illusion, as the strapped mono cartridge presents the high frequencies very well indeed, while the rest of the frequency spectrum is fuller, more fleshed out and robust. Just more solid and realistic, with the high frequency information being presented as much more an integral part of the whole as opposed to in isolation.
I have to admit that I am biased toward line contact or microridge styli as opposed to conicals and really would not be that interested in owning a cartridge with a conical stylus at this stage, even to play vintage or modern monos. I've done 4 retips on Denon 103R's in the past after running the conical on that cartridge for 1000 hours or so, and although the Denon conical is very good and perhaps one of the best of its kind, it simply gives up too much in performance to a more sophisticated stylus profile in terms of information retrieval and high frequency performance, even with vintage records.
It is a subjective hobby, but there are obviously some manufacturers of fairly high end mono cartridges (Lyra and Ortofon come to mind) who also believe that a more sophisticated stylus profile is beneficial in mono playback.