You would have to be beyond retarded to go into the military as an E-4 with the level of education a law school grad has.
I'd say it's an unusual choice, but not necessarily a bad one. It does help you get rid of debt. If you've just hung out a shingle and are starving while trying to establish your own firm, it would give you a way to get health benefits.
The other thing is that some people sincerely enjoy it. I joined the reserves after 9/11 because I felt the need to do something to serve the country. However, I am doing it today because I genuinely enjoy it so much.
Granted, if you get that biglaw job at $160,000, they're not going to be very tolerant of you having a part-time job, regardless of what it is, but I don't think the OP is in that boat.
Also, let's be frank, here. There are a lot of law schools out there and a lot of folks who attend will NOT be working in the law. The military is a viable and relatively well-paying career. It doesn't pay what biglaw pays, but frankly, your garden variety logistics officer probably has far greater earnings potential, a monumentally better retirement system, and considerably better benefits than a person who graduates in the bottom 3/4 of their class from a T2 school or worse.
Granted E-4 pay isn't that great, but an E-4 will still make about $3,000 a month, (1/3 of which is allowances, which are non-taxable) with excellent benefits and 30 days a year off. Plus, he'll be getting a heck of a lot of his federal loans forgiven.
I would venture to guess that 1/4 to 1/3 of the brand-new attorneys graduating this year won't do much better.