I think "knowing it's there when you need it" is part of the appeal of the 550i. Chrischeung's point on the status department is on the money: for some who want to show status, this means 550 numbers on the trunk. I might add, that for others who don't want to boast as much, it means 535 numbers on the trunk. Entirely personal choice on the numbers.
I always wanted the biggest engine as so many cars were underpowered previously for me. Now, they got the engine technology so good, the inline 6 is more powerful than previous v8's and gets better gas mileage than my E46 330i. There is sure some elegance here.