Like really, really bright! 

Why?  How 'bout that simple capitalistic reason - your bottom line.  You want those riders you pay to wear your advertising to be riding the front, or better yet, off of it.  That takes serious conditioning.  To get there, your riders have to stay out of the hospital (and morgue).  Best if they don't get hit by cars.  So MAKE THEM VISIBLE!

Case in point - Chris Horner, probably wearing his dark Lampre outfit, gets hit by a car in a tunnel.  Gonna cost him, his team and his sponsor most of a season of advertising.

Horner himself takes some responsibility here also.  Good, bright rear flashers are out there, not expensive and mount and come off any bike easily.  They cost $40.  Yes, I know, any self respecting pro would never use such a dorky thing.  But you are a professional.  Don't you  have a little responsibility toward your employer?

Vent, vent.  From "Mr. yellow jersey", a rider named Ben