Having 'toured' the various scales in my lifetime with this hobby, I keep relating this sort of question back to my HO days...
And in the world of HO I am reminded that a 18" radius curve is equivalent to an O72 diameter curve in the world of O3R.
I still have an overabundance of HO passenger cars squirreled away for another day that will probably never come again. They're all of prototypical length....as are the GGD cars. And a majority of the manufacturers boxes/specification sheets for those cars has the caveat, "These cars require a 24" minimum radius curve", or some variation thereof.
A 24" radius curve in HO is comparable to an O96 diameter curve in O3R parlance.
And, it's not just an appearance issue....Melgar's photos, above, show that very well. More importantly are operational issues....like backing up a string of 21" cars through O72 curves...or achieving a coupling maneuver between two cars on the curve....notice the last two of Melgar's photos to see the 'stress' an O72 curve puts on the couplers.
Frankly, if it weren't for the oversized flanges on the O3R wheels, a back-up maneuver would probably leave the string of 21"-ers scattered all over the track. Again, drawing on about 50 years of experience with HO, truck-mounted couplers (sometimes referred to as 'Talgo' in HO) are not at all preferred...or even 'allowed/accepted'... by the folks who are into the operations side of model railroading. Backing maneuvers, in particular, are essential in operating sessions. Body-mounted couplers...and proper weighting, among other qualities/features of the cars...are requisite.
And, yes, I know that most of the 21" passenger cars have body-mounted couplers with design features allowing a wide swing for sharp curves. But the alignment of these cars on an O72 curve during a back-up maneuver still results in a hefty force vector that tends to push a trailing car to the side, encouraging derailment.
I know that space to accommodate an O96 minimum O3R curve is a luxury for many home hobbyists. But I have to agree with John (Rattler21), above. And, since most everything else is a compromise in the O3R hobby segment, I limit my passenger car length to 15" in my O72 minimum layout....for aesthetics and operation.
Just one man's opinion, of course. TEHO.