You need to consider what school you're talking about. An MBA from anything less than a Top 20 is almost worthless. I thought about a JD/MBA, but decided not to do the combined program because my school's MBA program is not even ranked. I'm at a T-20 law school, and I'll likely be starting at a T-5 MBA program right after I graduate from law school. I know it's a lot of school, but an MBA at anything less than a T-20 is a waste.
What. Ever. From what vantage point are you deciding that a less-than-20th ranked MBA program is worthless? If you want to gain the knowledge that an MBA offers to broaden your knowledge base, then get an MBA wherever you are going to school. If you know EXACTLY what you want to do after college, then ask people in that field whether an MBA is worth the time. If you have the extra year and can afford it and have no idea what you want to do and have a curiousity about business, go for it.