[As an aside: I know numerous students from non-top 50 schools without grad programs who have gotten into top 20 and top 50 grad schools without Masters, so don't let that stop you from doing anything]
I have no special knowledge on this program, but I wanted to offer some encouragement because this program seems like a smart move. If you are interested in Philosophy of Law then having a JD is possibly going to help and not hurt (except for the time/effort/money it takes) your chances of later getting into a PhD program to study this topic. You've said it presents a possible fall-back if you don't get into a PhD program, but it also creates a possible fall-back for when you do get in to a PhD program but then can't get a tenure track job. You would be qualified to practice law full time; to perform legal consulting, serve on ethics boards, etc. during the summers if you are teaching; teach Philosophy of Law at a Law School, which would still afford you time to research/write, you'd be able to teach high caliber students, and presumably you'd be at a university/college that also has a philosophy department you could interact with.
Obviously it would serve you better to be in the PhD/JD program, and perhaps you should turn it down for a PhD from a good school, but it seems like this is a better option than an MA in Philosophy from another school.
Just my opinion. Good luck!