I was a finance/econ undergrad at Texas (actually transferred from Bama - roll tide) and am a law student there now. I applied and was accepted to the JD/MBA program here and was strongly considering doing it, just like you.
After countless hours of research I decided against it. If you want to practice law, even in M&A, you don't need the MBA. You would be surprised at the number of attorneys with zero business background that practice in very business-centric areas of the law. Your undergrad education alone with give you a leg up, and if your future law school is anything like mine, you'll be able to take graduate courses at the business school if you want to refine your knowledge of certain areas. Plus, MBAs aren't really intended for people who don't have significant work experience like me, and I'm guessing you.
As for the negatives, it's costly, time-consuming, doesn't add much to your marketability in the legal market, and may actually deter prospective employers.
It's costly because you're almost certainly not going to be eligible for scholarship consideration entering the JD/MBA program, while traditional applications applying solely to the MBA program are. Three years of law school is already insanely expensive, and adding another year of full tuition and living costs on while simultaneously sacrificing salary you could have been making is a pretty big financial hit.
As for marketability, I discussed above why it's not going to add much to what you have to sell as far as business acumen as a first-year associate. Further, I have heard that firms could be skeptical of whether you are truly committed to the practice of law or whether you're going to jump at the first chance to run off with a client or some other greener pasture in the business realm.
The only circumstances where I personally would consider doing it would be if I had struck out at 2L OCI and wanted the MBA year to essentially get a re-do. I know people who have done this and been successful. The second situation would be if I went to a school that had both an incredible law and business school (e.g. Penn, Harvard) and I was in a good financial situation. It would be even more of an appeal if I were at a school like that and was interested in management consulting, as a JD alone, much less with an MBA, would place you in a pretty good spot with the Big 3 MC firms.
Seeing as how it's highly unlikely you're in one of the situations I described in the paragraph above, my advice would be to forgo any JD/MBA program. Best of luck.