In response to Harrumph, you bring up some valid points. However, as a content creator, I prefer sampling analog game video as accurately as possible, regardless of how good it necessarily looks. I want footage that is as accurate as possible for my reviews, and while I personally prefer the deblurred N64 settings, it isn’t accurate to what the system is outputting. Personally, I think pixel accurate N64 looks nice enough, if the more accurate Angrylion Project64 plugin is anything to go off of. I’d upload some photos, but I can’t seem to get the forum to accept any of my links.
Plus, having a dedicated optimized mode will make it easier for users to sample their N64, just like the 256×240 mode streamlines that process for NES and SNES. I also find that displays are more tolerant of the OSSC when non-standard horizontal resolutions (like 512) are sampled within a standardized H Active (like 640). This means that the 640 mode could have additional applications for sampling games that run in anamorphic aspect ratios while keeping an aspect ratio closer to 4:3. As Nmalinoski also notes, there are also games that natively run at 640×240 which could use this option.
Regarding the issue of whether Generic 4:3 interpolates, I heard that from a friend who also owns an OSSC and is generally pretty knowledgeable, though he may be mistaken on that point. While I can get pretty close to the image I’m looking for in 1280×240, the obsessive part of me just wants to get that extra bit more accurate as well as the ease of use.