It's obvious Kemer is using a Paracorr 1, which is no longer in production. The current, Paracorr II, has greater tunable top adjustability, and all 3 of the Ethos eyepieces he mentions focus at the same point.
Coma is more visible the wider the apparent field of view, so the use of narrower fields (Delos vs. Ethos) may make the Paracorr somewhat optional at f/4.5-f/5, but, shorter than f/4.5, not using one just means you are putting up with less-than-sharp star images.
The coma-free fields of view in telescopes are as follows (remember, 2" = 50.8mm)[.01778mm x f/ratio cubed]:
f/6---3.84mm
f/5---2.22mm
f/4.5---1.62mm
f/4---1.14mm
f/3---0.48mm
Now, coma doesn't spring into being at that point--it's a linear function that grows steadily worse with increasing distance from center. The above figures represent the central areas of the fields of view wherein the comatic image is still smaller than the Airy disc, and so, invisible.
It does point out how little of the field of view of that 21 Ethos (36.2mm field stop) is free from coma in Kemer's f/4.2 scope (1.32mm coma free field).

I like to look around the field of view a lot and see what's there. I don't really appreciate it when the stars in the outer half of the field look like small Perek-Kohoutek planetary nebulae.
The Paracorr is expensive, and certainly adds weight (you need counterweights on a dob anyway, so this is a red herring). Awkward? Not so much. Extra glass? Surely, and it probably causes a loss of 2-3% of the light in the scope, which is a profoundly lesser amount of light loss than occurs with the accumulation of 3 months of dust. And do you clean your mirror every 3 months? [If you saw the statistics, you might]. Probably not. Well, worrying about the light loss in a Paracorr (which I did prior to using one) seems to be one of those things in life not worth worrying about since the fainter stars that became visible in most of the field because the Paracorr was there overcame any objection I may have had over whatever light loss might be there.
A driven scope looking at planets centered in the field of view? Don't need a Paracorr, even at f/4.
A non-driven scope looking at objects bigger than planets? Definitely worth consideration.
Don Pensack
Los Angeles