It is a love or hate design.
I used to think the same way Phil, why do they need it. Then I watched a documentary on models and cars, does the name/number matter. Turns out it doesnt most times, a bmw wanna be owner will buy a bmw of any series if they "like" it. So if they pass on a a 4 door 3 5 or 7, they now get a 6. Evenetually buying one and everyone is happy.
Obviously it isnt that simple with guys like us but most bmw or merc owners are clueless about cars in general, they just know they own a bmw or merc.