Soldato
It's only a waste of money until you need to claim, just like regular insurance on anything else.
On the warranty debate... not a Z4, but I've had my car (e92 330d) around 6.5 years now and so have paid approx £3-3.5k out on warranty subs in that time (first year free as AUC). I estimate the car has had round £20k+ worth of warranty repairs during that period, which is almost what I paid for the car in the first place.
As a value proposition, I'd say it's one of the best decisions I've ever made. Sure, if I had taken the car to an independent then some of the bigger jobs might've been a bit cheaper, but also some of the smaller jobs might've never got done and I dare say that it would've still cost me more than that £3-3.5k for all the work that has been done.