I'll chip in here with a reason for good, visible numbers -- to help Ron Christensen, the radio announcer, see which vehicle is at the line. I've spent a few hours in the tower with him, and even with binoculas it's often difficult to read the number on a bike (car/truck, too). I know that color and design scheme have a lot to do with the display, so try to keep ron in mind when you're putting numbers on your vehicle. It helps him make a more interesting broadcast.
As for number "plates": I've seen more racers trying to bend that part of the rule to their own needs - some potential streamlining effect, mostly - than I can count. Lots of hassles, lots of stress -- all for strict compliance with that rule. Nobody's ever complained about the numbers on my bike -- which wears its' numbers on the tailsection, right on the blue bodywork -- no white 6" x 8" white background or anything like it.