Yeah, it all depends on how safe you want to be. When I was a kid I would talk my Dad into letting me ride in the bed of a pickup truck when he'd borrow one from work to take something to the city dump, like an old washing machine or stove.

Not that I'd recommend that to anyone...

You have to keep in mind that those recommendations/instructions are vetted by lawyers to minimize any potential liability for BMW. So it is difficult to determine what the real risk is, unless you can get some crash data from the government test and/or insurance organization crash testing, as they might use crash dummies in the third row and have some data/guidelines.