Keep in mind that police tactics really don't emphasize the bashing/striking/clubbing with the batons of any kind -- they're used most often as deterrents and only on rare, extreme occasions are they used to hit someone with (they work extremely well, however, as a lever when performing armlocks and arm-bars and other restraining-type moves).
That being said, you don't have to have a lot of weight -- they hurt like hell. They also have a nub on the end (like a half-flattened-ball) that is a focus point for all the weight and speed of the swing, so they do work very well when used as such.
I think the main reason police departments went from the traditional tonfa-baton to the collapsible baton (or ASP) was for two reasons -- Training (or lack thereof) and size. They didn't have enough people who properly knew how to use the tonfa-baton and didn't want to teach them, and the ASP is so much more compact and easy to carry that they just made the switch.
