Performance aside, if you've got the money, why would you choose a Nissan over a BMW? I'm not talking about 370z vs _____.... so fanboyism aside: Talking about cars we don't even own (well most of us on here don't have maximas or 5 series)
Someone offers to give you the keys to a fully loaded BMW 535 and a Fully Loaded nissan maxima, you'd choose the maxima?
or a 1 series vs a versa? or a Murano vs an X5. seriously
money aside you'd choose nissan all the way?
I can totally see why someone would want a 370z over a 3 series, but overall brand quality, money aside, come on. Theres a reason that bmws cost a lot more than nissans.
and to answer your question. aside from totally random vandalism, usually if someone keys your car its because you pissed them off somehow. Whether you stole their spot (or so they think) cut them off, parked too close to them, or simply have a nicer car, you bothered them. So if you want to decrease the likelihood of having your car keyed then its best not to piss people off, and all your plate does increase the likelihood of that happening
The point of vanity plates is to be clever, cool, or funny, or some mix of it all. Its all fun when someone says 'oh haha i get it!, great plate!' or even some sort of little rivalry like Supraklr or something. The only comments you'd get from your plate is a middle finger from a bmw driver, and maybe some other douche in a parking lot coming up to you and telling you that he agrees and doesn't like BMW either, talk about a good time.