Extended warranties are massive profit centres for dealers and manufacturers. Terrible investment to make and any advice to the contrary is bad advice.

Third party warranties may make sense as they typically offer better value and are underwritten using actuarial principles and not based solely on what the market will bear pricing. 

That said, a reliable BMW is reliable for a long time. Once it starts to become unreliable, it is expensive, but I know of no one who owns one who has had any problems within the first 8 to 10 years and 100,000km.