Well, I have read that the R/C changes the primary-secondary spacing and this increases spherical aberration.
It seems there is a lot written about the R/C and what it does or doesn't do - but not a lot of agreement. Without getting too scientific or name-checking any aberration types, I can only say that the views through the R/C are just plain better than without. It helps that the magnification is lower of course, but the R/C seems to give cleaner images at matching levels of magnification*. I like it.
@Don Pensack - 1.4° field of view is more than necessary for most DSOs, but it is not just a matter of framing. A wider field helps with me finding the objects, and given that I use a manual and pretty basic AZ mount, a wider field makes life far easier. Also, aesthetically, the field of view and the quality of stars delivered by the C6 + R/C + ES20/68° are very nice - I think the field flattening of the R/C helps with the widefield eyepiece, and if there is any added SA, it's not visible at 50x power.
Having said that, I have tried the R/C for planetary viewing (a couple of years ago) and I recall I had enjoyable views... So it seems to work at high powers too.
*Regarding matching magnification, I have the perfect R/C tester eyepieces: my Speers WALER 10 mm and 13.4 mm 82° eyepieces. They come with an extension tube that gives 1.6x zoom - which almost perfectly compensates for the 0.63 unzoom of the R/C. 1.6 x 0.63 = 1.008.