The Vault has an interesting article about joint JD/MBA degrees. In it, the value of having both degrees is questioned: Does the added investment required to get the second degree result in a satisfactory return on investment?
Getting a joint degree most likely requires a full-time commitment that will not be reimbursed by an employer, so a student most likely will need to decide if the additional education is worth making the additional personal investment.
The article notes that not only may a second degree not be beneficial, it may even be detrimental to a job search, because potential employers may percieve as dual degree holders as not being sure about which field they desire to pursue.
Santa Clara University offers a dual JD/MBA degree from its Leavey School of Business and School of Law. In my five years at SCU, I never was on a team with anyone pursuing a dual degree, so I don't think it is that common for students to go for that option. So for a particular postion that could use both JD and MBA skills, having the dual degree could be a major and rare edge. Just how common these types of positions are is the important question.
Here's the article in Vault: Ask Anna: Are Two Degrees Better Than One?