I used to cross-drill brake rotors in the early 1970's for Ducati 750GT motorcycles (180 holes in a 3/2 repeating pattern). The reason was twofold: to lighten unsprung weight and squeegee off water in the rain. Hole diameters should be equal to or less than minimum rotor thickness for solid rotors. Pad wear did not increase noticeably, but once drilled the brakes make a "chirring" sound when applied. IMO definitely vented for front and cross-drilled for all.