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A lot of wishful thinking in that statement.
The cancellation would only occur at one single telescope's focal length (about 500mm judging by those graphs); and only for ONE Petzval curve (saggital and meridional curves are quite a bit apart, indicating eyepiece's residual astigmatism).
In my experience, no Pentax eyepiece gives sharper images at any given distance off axis than a corresponding Nagler. Naglers are significantly wider, sharper off axis, smaller (type 5 and 6), work better at fast f-ratios and can completely take care of residual astigmatism of the observer's eye via Dioptrix, if needed (eliminating the need to wear glasses while observing). I can see people liking Pentax eyepieces, but for me they hold no interest whatsoever. They may be a smidge more neutral in tone, but that is about it.
BTW, Newtonian's field curvature is rather unimportant (unless you use 8" f/4 or shorter as your main scope). At 14" (or 18") at f/4.5 telescope's curvature is negligible (10mm off axis, the curvature would amount to about 30 microns, worst case. At that amount, defocus blur is barely larger than Airy disc, and is completely swamped by coma).
I think a lot of people prefer large eyelens because they use eyepiece as a screen (to look AT, as opposed to THROUGH). I actually find huge eyelenses annoying and rather distracting.
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Bratislav,
You can sit here and type whatever math you like, but the eyes do not lie. I have used Naglers and Pentax eyepieces for years. The off axis distortion in the Naglers I tried in my 10" f/4.7 was all too evident as compared to a 10mm, 7mm, 5mm and 3.5mm XW. The same goes for the Pentax XL eyepieces in those focal lengths, I am not talking coma here, I am talking AMD and RD. Off axis, these Pentax eyepieces show a lot less distortion than the Naglers I have tried.....which were 9mm and 7mm T1 and T6. I see ZERO distortion in any of these I just mentioned. The Naglers have a lot of rectilinear distortion and AMD off axis. The Pentax eyepieces I mentioned showed ZERO RD or AMD.
FYI, Field curvature is there in my 10" f/4.7 scope in case you were wondering, but I see NONE in the 10mm, 7mm, 5mm or 3.5mm XW's or 10.5mm, 7mm 5.2mm XL's.
Don't get me wrong, Naglers are fine eyepieces indeed in certain focal lengths, but what you said above was completely wrong.
Just sayin'