The series of pre-flash are meant not only to be an AF assist but also serves to calculate the proper exposure at the time of a flash during the real exposure. So you get a more accurate exposure when the flash is fired. The pre-flash method has always been Canon's style and if I'm not mistaken its part of E-TTL II. If you're using speedlites like 430EX or 580EX, the pre-flash goes on as well apart from the AF-assist beam.

Its also good to know that pre-flash does not happen all the time (depending on the light meter), so it cannot be treated as a method to reduce red-eye effect. Take note that pre-flash only happens when you half-press the shutter button and not during exposure. Thats when the camera calculates the exposure and perform AF. Further pressing will release the flash and capture the exposure. Using speedlites mounted high above the lens-to-eye axis is a good way to reduce the red-eye effect. Strobe flashes would also help.

I got this information from the EOS Lighting Sytem: Field Guide of 430EX & 580EX. Its a book available at Borders, MPH and KinoKuniya. Hope that helps.