I don't wish to speak for everyone on this forum, but Big Law just doesn't seem like it would work for most of us. If I went into law, I'd want to work as little as possible, go home every night, and read philosophy. 80+ hour work week just doesn't go with the philosophical life. So, personally, I wouldn't factor Duke's employment statistics into this decision.
There are two things I would think about here though. If Duke is funded, you're going to need to be in the top 50% (or so) of your class to keep that scholarship. Remember: Law students take far more classes each semester than philosophy M.A. or Ph.D. students. I would be concerned about how many classes I would have to take each semester, which would make me worry about my performance in those classes and thus my prospects for Ph.D. admission. I think you do two years in the law school, one year in the philosophy department, and one year in both, in classes that count towards both the law and philosophy degree. It's four years; it seems rushed — it is rushed. I would be worried about keeping a 3.9+ in my phil. classes if I had to take more than four courses per term, which I assume at some point you will have to do.
That said, the ability not only to teach 101 classes throughout the Ph.D. but also to go to traffic court on weekends—make a couple hundred bucks here or there—seems really attractive. If, say, you want a family down the road, it might be manageable while in a Ph.D. program if you could supplement your stipend with both adjuncting and doing traffic court or simple criminal defense.
Regarding ultimate placement, I know people strike out, but it is less likely the more degrees you have (not hard data, just my experiencing in googling people's placement). I'd feel pretty secure with an M.A., J.D., and Ph.D.