Adding an MBA to your list of academic accomplishments can only benefit you in the long run, no matter what you eventually choose to do with your life or legal career. Since you seem to have decided that joining a law firm is what you want to do in the near future, there are several benefits to having an MBA when entering law firm life.
First, given the type of training that occurs, in our opinion it would be quite wise if an MBA were an actual requirement for getting a law degree. This is due to the fact that the practice of law is so closely related to business and few attorneys appear to realize this. In fact, attorneys traditionally are known in the business community as some of the worst businessmen. Being an attorney has numerous components that involve business such as marketing, the value of your time, hiring and firing, renting office space, managing payroll and numerous cost-benefit type calculations. While traditionally law was portrayed as something that was not a business, you should make no mistake about one central fact: The law is a business.
If you start your career in a law firm position, you may one day become a partner and be responsible for many of the day-to-day business decisions associated with the running of the law firm. Even if you do not choose to remain with a law firm, you may one day open your own practice. The training an MBA provides will help you navigate the waters in terms of running your own law firm. Your own law firm, incidentally, would be a small business.
A second aspect to consider in getting an MBA is how it could teach you to think in a different way. MBAs are known as "bean counters" for a reason. As a general rule, MBAs tend to be very risk averse and good at pointing out the risks inherent in any business situation. In fact, many entrepreneurs that have hired MBAs report that they are continually told by MBAs to simply shut down their businesses! This type of risk averse behavior is exactly what most lawyers do with clients on a daily basis. While we are, in fact, having some fun with the above statements, the generalities they express are on point and some of the better attorneys we have known have been MBAs.
Third, you may consider getting an MBA if you are interested in corporate work. Beyond the practical training and the training in the "MBA thought process", an MBA will also teach you a great deal about the inner workings of the financial system and about many of the types of companies you are likely to do work for as a corporate attorney.
Fourth, even if you decide not to do corporate work, an MBA is not a liability. Since the majority of law schools with joint programs heavily favor the JD degree when constructing the curriculum, the amount of legal education will be on par with that of any other student who is just focusing on the law. Additionally, any extra knowledge that you can bring to the table, even if it is not directly applicable, is going to make you a more attractive candidate.
Fifth, if you eventually decide that practicing law is not for you, having an MBA is probably the only other degree that instantly grants you access to the kind of high-paying professional jobs that lawyers enjoy. Make no mistake about it: numerous attorneys leave the practice of law each year. You may be dead-set on being an attorney right now, but things change and many people who go into the law eventually come to rethink their decision. An MBA is a great thing to be able to fall back on.
The only con - and it potentially is a big one - is that some law firms may interpret getting these two degrees, which really represent two diverse career paths, as indecision in what you want out of your future. If a firm thinks that you are applying to them simply to get a feel for what working in a law firm is like before you make the jump to corporate life, they may be less willing to hire you. If there is one thing that law firms value, perhaps above all else, it is a singleness of purpose when it comes to careers and what attorneys are willing to give back to the firm. With that said, there are many firms who are not nearly as concerned with this, but the stodgier, more traditional firms may not understand your motivations.