Something that's been picking at my brains...

[lincpimp]

If you have ever used a cordless drill with a flat head tip to try to drive a screw in or out and could not get the bit perfectly perpendicular to the screw head you will see why flat head screws are not as popular...

You can drive a philips screw with the bit at an angle to the screw head.

Also, using a power driver the slot of the screw tends to "throw" the bit out at high speeds. Pain in the rear, I would be glad to never see a flat head screw again. Plus the tapered slot always wants to force the bit out under load and that breaks the bits and messes up the screws.