I'm considering this too but I'm getting conflicting feedback, including from those already seeking joint degrees.
Some say there's no real point to it, if your plan is to work as a lawyer. Those people say there is little of benefit you'll glean from the extra degree over what you will learn in business law classes. Also, most lawyers don't have an MBA, so they might be suspicious of anyone they interview with other/"better" qualifications - it may demonstrate a lack of focus ("do you want to be a lawyer or a business exec?"). This seems absolutely silly to me, but I've heard it from more than one person. I'm of the school of thought that the more degrees you have the more awesome you are, but I guess not every one thinks so
If you are a member of an under-represented class, it might make you stand out to have the joint degree. Meaning not just URM but also women. There are very few female JD/MBAs and it might give a boost.
So in sum, I have no idea.