I don't see why a reducer would be any worse than using an eyepiece for afocal and most imaging reducers have broadband coatings that should get you pretty good transmission into near IR.
The C mount NV device will fully illuminate the photocathode at speeds up to about f/4 or so when used at prime focus.
Now it will be a matter of personal preference, but I find that even at f/2.8, for visual use, the illumination falloff is not objectionable. You can though easily see the illumination falloff at f/2.8 in a photo.
Since the vast majority of refractors cannot be reduced to speeds faster than about f/4.2 when using imaging reducer/correctors, this should not be an issue, but faster than this and the fully illuminated field starts to shrink, but again, it is very subjective as to where the cutoff for that illumination would be.
So, I don't know why you would not want to use a reducer other than the fact that simple reducers usually have poor off axis performance when coupled with faster refractors. A good imaging reducer with field flattening might not give you the speed of some of the simple reducers out there, but the loss in focal ratio can be a very good compromise for people that like a high quality view out to the edge of the field. I don't get a fully illuminated field in my 6" f/2.8, but the field is very sharp, and the vignetting is so minor that it does not show in visual use but the camera sees it.
Anyway, there may be subjective reasons for not wanting to use a reducer, but it does work in that it does decrease the focal ratio.
Edited by Eddgie, 23 October 2019 - 09:01 PM.