I'm not sure I can agree with that totally Ken....I do to a point..12.5" newt as example.
When it's PERFECTLY collimated, it doesn't seem to matter what quality EP I bung in, weather it's a .965" Huyghenian, Ramsden or a 1.25" UO ortho I get absolutely great views! I'm talking view's that would stun you Ken..Iv'e gone to the extend of purposely putting the scope out of collimation to see what would happen to the view's using these 3 particular EPs. mentioned above.. The result was obvious straight away, Couldn't get a sharp image no matter how much I tried focusing. The 'soft' look.
Now after saying all that, I agree some EPs are total crap! I have 2 of those 500s. The view's I get through them are actually pretty sharp as long as the object is dead centre in the FOV..A lot of flare & ghosting, but if one can look THROUGH that, a pretty sharp image. Put the scope slightly out of collimation though, & it's instant crappy view. Naturally seeing & transparency etc etc all come into play as well.
Anyhow, that's my experience that Iv'e had with my particular scope...Perhaps My Parks mirror is better than what I thought!
?