The ATX mount means that most of the lower left side of the case is solid, no? (The ATX motherboard trays covers up most of the lower left portion). Even if the right hand side panel is fully perforated, you'll just see the back of the motherboard tray instead. Perhaps the motherboard tray itself could have some cut-outs made to the lower portion, but then you compromise on the strength of the case (and visually it won't look very good from the left side since you'd still be seeing the back of the motherboard tray through the side panels at the positions where there are no cutouts)
There are sufficient cutouts on the lower portion of the motherboard tray to allow at least partial ventilation for the followthrough cards. In terms of looks the already perforated part of the rear side overlaps solid areas as is. I don't think it looks bad given they are painted the same colour as the outside. If you compare the two areas in question they are really not very different.