How are you playing your precious MONO Vinyl?

This subject has been rehashed many times.  Many of the mono cartridges on the market are nothing but stereo cartridges in which the two channels have been bridged internally. The mono switch on a linestage also achieves mono output into both channels by virtue of bridging the two channels at the output.  So, when comparing this type of mono cartridge to the mono switch, the only difference is that in the one case the two channels were bridged before RIAA correction and amplification; whereas in the other case the mono signal is created after these processes take place. I could imagine reasons why either one is better than the other.  The other factor is that when you use a mono cartridge and compare it to your stereo cartridge, unless you are using the artificially created mono version of the very same stereo cartridge you are otherwise comparing two different cartridges.  Any two cartridges can differ in sonics by virtue of many factors, so it is hard to say in that case that "a mono cartridge is per se better".

@lewm 

The Lyra mono cartridges (Kleos, Etna, Atlas) are true mono cartridges. They are not strapped versions of the stereo cartridges. Dynavector also makes a true mono version of the XV-1s. 

For about $300 one can dip their toes into using a true mono (not strapped) cartridge with the Audio-Technica AT33MONO.

I hope I did not say there are no "true" mono cartridges. I only refer to the type described. Still, the second caveat for comparing is valid; your particular mono cartridge may just be "better" or "worse" than your particular stereo cartridge. Therefore any conclusions are subject to that qualification. I do think even using only a mono switch when playing mono LPs with a stereo cartridge is superior to not using the mono switch.  At least in that comparison I am not using two different cartridges. If you want to get really purist, you can use one of those few mono cartridges that only has two pins and use it to drive only one channel of your stereo system. I've never tried that nor heard it on someone else's system.

@lewm 

I think the stereo/mono equivalence for a comparison between my two cartridges is quite valid. They are both Lyra Atlas, i.e. the same design. It’s just that one has stereo coils and the other has mono coils, and if you look at the design diagram on the box of each they are exactly the same. Only the coils differ between them. Otherwise they are the same cartridge (i.e. design and build). I can only tell the difference between them visually by looking at the serial number.

With the Lyra Mono one can also connect the two mono channels in series for additional output when used in a pure mono system.

See here what Lyra (i.e. @jcarr) has to say about it: https://lyraanalog.com/atlas.php (you’ll need to select the Mono link on that page)

I had Steve Leung of VAS convert an older BM Ruby 3 stereo cartridge by rotating the former holding the coils 45° and connecting the horizontally sensitive coil to both outputs. He removed the disconnected coil and put a new stylus on it. This was a lot cheaper than buying a new mono cartridge. But back to the original question:

Is EQ-100 or something similar, absolute necessary from a purist perspective or should I take the pragmatic path and use the ‘Mono’ switch on my Integrated with a built in phono?

If you have a true mono cartridge there is no need to use the mono switch on your integrated amp: each channel is receiving exactly the same signal, so it should make no difference. As to whether there is an advantage to using a mono phono stage, that is a matter of the relative quality of the phono stage you use now versus the proposed mono stage, but I don't think there is a reason to say that a mono phono stage is always better than a stereo stage used with a mono cartridge. If a purist approach is to be taken, with a mono amplifier and a single speaker, it would conform, but that involves a whole separate system and I don't think that's your intention.