rjek: "but modern hard discs kick out lots more heat"
You obviously don't remember full height drives and ST506 models, a modern drive uses a fraction of the power, and hence heat generation.
Modern drives don't need much cooling or direct air flow, just that the ambient temperature is kept under control within the case. Without the several hundred watts generated by processor, northbridge, and ridiculous graphics card to have to deal with, a small case fan is adequate in most RISC OS machines. The exception being heavily expanded multislice Risc PC's, which may require a second fan.