The wire you see wrapping around the antenna and running the full length under the black plastic coating is used for Asian cell phones. Cheaper to build one model antenna than to make them market specific, or they had a bunch of old crap left over and cut a corner...
I have BMWs sport stubby on the Z3, haven't bothered on the Z4. The only reason I'd change it is down to it having a useful knack of poking you in the eye when washing