My argument against setting any denoiser as a default would be that most likely you would get flickering when rendering sequences.
Corona defaults are supposed to be safe and with as little bias as possible while keeping reasonable render times.
Also, consider these posts:
There are limitations though which aren't obvious:
- Complicated shader trees tend to resolve detail (bump, procedural details in noise maps etc) at a later stage in the render progress. With only a few passes the result will be less surface details.
- Some of Corona's sampling-based maps like AO and Curvature will also refine at later stages. I got some heavy differences between the two images where AO was much less pronounced and detailed in the downsampled image (with less passes). If you're using those kind of maps, it's best to still allow more samples.
- Refraction details also tend to take a bit longer, so you trade sharper details (implying more detail) for less actual detail and accuracy.
(...)We're finding significant detail boost at lower passes using this method, and that's without actually rendering at 2x scale and downscaling back down.(...).
So maybe that video isn't as revolutionary as it sounds. :)