How to sell your "Why do you want to do an MBA"?

But for the outlier, extremely focussed applicant, everyone else must rack their brains, face down their demons and arrive at an honest to god (cross your heart) answer which forms the basis of your application strategy.
The deal however is, the more people's views I get to know about it, the more convinced I am that honest truth is not enough to get you through.
If god forbid, your stated reason to do an MBA is that you are a generalist, or looking for a classy career improvement training, your slot will in all probability go to someone who claims to have a passion for derivatives trading and is looking to become a director of a european union's pensioners fund over the next 5 years, a George Soros over the decade.
Seems alright to me - why not give the limited slot to someone who knows where he is going, instead of to someone whose career planning is still very vague.
The point is, "Why MBA" needs its share of selling (again alright in my view - you must know how to sell before you can claim to be ready for business), and so, a lot of polish. The god honest truth needs to be propped up by thorough research and a commitment to some kind of a career position - short and long term.
Firstly - I want to get a consensus if such is indeed the case.
Secondly - I think there is a possible flip around for people who are grappling with the question. Flip the situation around and ask "If you got an MBA, what would be your dream job then?" from the options choose the best one which requires experiences similar to the ones you've had. I speak of it with caveats, without having applied it myself - but from where I am ( a generalist seeking ways to solve high level problems) I see no other useful tool.