There's a pretty lengthy article about whether playwriting can be taught by Davi Napoleon that's just come on-line. She talked to tons of people (including me) about it, and it seems to cover a lot of the angles.
In addition, Malachy Walsh, one of my favorite bloggers is finally back on the scene after a long absence, at LitDept. He has two great posts about the value of an MFA for playwrights, one addressing common myths about playwriting MFAs and the other about the dollar cost/value.
For a long time, I was pretty anti-MFA, though I've considered going back for a master's degree several times. The time never seemed right, or the program never seemed right for me. But I've softened my stance considerably over the years, especially since a lot of my best friends have playwriting MFAs and even teach playwriting in various programs. For some people, it's the best way to learn. For me, I had to learn playwriting on my own, by running theatres, writing plays, screwing up, having some success, banging my head against the wall (a lot). But that's just me.
Maybe I would have learned everything I know now about theatre a lot faster in a graduate program, and I'd be much better read. Hard to say.
From my point of view, borrowing money to get a graduate education in playwriting would seem a dicey call, however. It's almost impossible to earn the money back in a timely manner by writing plays, and the cost of the debt would seem likely to suck away the time after graduation needed to actually make use of the skills gained in graduate school.
I've been pretty lucky in having a spouse with a full-time job, who has been extremely understanding of my writing life (i.e. has not demanded that I go get a full-time job). I've made some money working part time, fixed up and sold houses, and I also have been a stay-at-home dad with our kids (going on 15 years now)--all of which took plenty of time, but also left me with more time and energy to write. So if the point of going to graduate school is to get dedicated time to write, maybe I never needed that, because I've always found a way to make the time and usually had the discipline to make use of it. The place I feel I've most missed out, in terms of graduate school, is having close mentors to whom I can turn to ask questions or help guide my career. I spend a lot of time fumbling around career-wise, and also working a lot with my peers (rather then people a generation ahead of me, in terms of skill and career).
For now, I think I'll stick with the road I'm on. But I can definitely see why others choose other paths.