Weight is weight, that's true.  There are two other factors at play though.  One is the friction of the water against the tube, which probably has some wrinkles in it as well, making it a little like a paddle wheel.  Also, the drag and friction of the water is well away from the center of the rim, which makes it even more effective.  Think of holding a bowling ball next to your chest and then spinning yourself.  Now drop the ball and pick up an aluminum ladder and try to spin.  Similar weight, but the ladder is much harder to get started and to make it stop because the weight is farther from the center.  Any force exerted on the end of the ladder is going to slow you down much faster than a force rubbing on the bowling ball.

 All in all, fluid in your tires isn't going to break you at the end of a day, but will pay for your beer on New Year's Eve.  I still hate paying the money for cast weights though.