Well, I can only answer for myself, but my reasons are kind of similar to Rhyme's. I just wanted to note that I don't believe I necessarily have more free time than others, the main difference for me right now is that I run my own business and I work from home. So, I'm in front of my computer all the time and I don't have a boss looking over my shoulder. I'ver never worked in a cubicle before, but I'd imagine that with the same current workload, I wouldn't be able to post even 10% as much if that were my work set-up.
Ok, now to your question.
I worked as a securities lawyer in New York and I always wanted to be on the "action" side of the deal. Granted, I may not need an MBA to do this. For example, I was on a deal with an Ibank where the director had a law degree and started his career as an associate with the firm I was at. But, it seems to me that while I probably have access to a specific department at 2-3 Ibanks, an MBA (from a top school) would open doors to many many more opportunities.
My next reason is that my in my current business, I have had substantial interactions and dealings with the far east. I am a fluent Chinese speaker and I believe that the future of business lies in working with China and to a lesser extent India. In the current economy it makes sense to produce products cheaply and import them to the US. But in the near future, maybe five and certainly ten years from now, I believe the real growth will come from delivering services to economies that are growing far faster than the US economy. For example, China is already a consumer of banking and legal services (I worked on the financing for the three gorges project as a law firm associate). It's appetitie will only grow. It's also clear to me that as factories and industries privatize and grow, they will become voracious consumers of management consulting services. So whether you are interested in banking, strategy, or some other specialty, being able to deliver your expertise to China (and maybe 5 years later to India) is highly desirable. I hope that with my legal experience and (newly learned) management skills I will be uniquely positioned to take advantage of these opportunities.
Finally, I often think about the idea that most people graduating from college will change jobs 10 times in their lives and change careers 5 times. Obviously, it's just a generalization, but there is some truth that people should think about. My law degree sets me up with jobs at any of the top law firms in the country, and could possibly be used as an entry into strategy consulting or banking, but flexibility is limited. I have spoken with friends that would like to work for some of the big tech companies here in the Bay Area, and those companies just don't know what to do with law degree holders. Certainly, there are openings in their legal departments, but they don't receive serious consideration of other types of jobs. In fact, a law degree probably hurts you if you are looking for a positing in marketing or general management with say, Google or Yahoo, because they are concerned with your salary demands. On the other hand, any large company knows exactly what to do with an MBA from a top school, and that will likely be the case in 5, 10 or 20 years. Can anyone predict right now wither Microsoft or Goole will be a more influential company in 10 years? Or will it be another company or an as-yet unknown company? Whoever it is, they will probably need people with management expertise.
Finally, most of the top MBA schools have cultures that promote strong alumni ties and relations. This simply is not the case with law schools, and alumni ties between undergraduates is superficial at best. If, when I finish business school, I decide that rather than join a company I'd rather start another business of my own, the ability to tap into a driven and qualified alumni base is huge. If I join a bank and 10 years down the road I decide I want to do general management in a different city (or country), a powerful alumni base could be invaluable. The best schools offer this type of access, and I guess you may never know how beneficial it can be until you need it.
So in terms of money and earning potential, I think I could do pretty well at a law firm (the partners I worked for in NY were taking home 3-4 million a year, the ones I worked with in LA over 1 million annually), but I didn't enjoy the work and I hated the rigid nature of the career path. An MBA from a top school (I hope) will let me call the shots with what I want to do down the road, especially since it isn't totally obvious what that might be at this point.