Originally Posted by tabetha

You've got it the wrong way around, the flow goes from bottom to top engine wise, ie opposite to the radiator where it enters at the top and goes down.
They are used primarily on very heavy boosted cars that develop hot spots in the water around the cylinder/jackets, this cases air to form, but so long as the return from the highest point is working ok, ie from the turbo this will return any air to the tank, so not really needed imo.
I think on the vast majority they are a gimmick, I would never have a metal tank of any make, as like yourself I like to see my level, don't try plastic welding the tank with another fitting, if you wanted to you can always "T" into the turbo return line with no problems.
tabetha