SFX folder contains mostly gunshots, explosions and other random effects, which won't be enough for many mods. For example, what kind of default sound would you use to indicate an airstrike? Or a start of a new zombie wave? Definitely not a random gunshot..
At least allow playing any sound file as long as it's in SFX folder. If sound is not found - skip it. Players will have to download a mod with proper sounds manually, but IMHO this is still better than being limited to default sounds.