While your earning a Ph.D. in economics, you learn two big lessons:
1. There's no such thing as a free lunch.
2. You are in serious pain.
On reflection, though, I've decided that both of these lessons fall short of the truth. In fact, a Ph.D. in economics is, relative to other graduate degrees, an amazing deal. It's not literally a free lunch, but it is a heavily discounted lunch. Consider:
1. What other Ph.D. can you realistically finish in 4 years? People in other fields routinely take six or eight years to get their degrees. Most economists don't finish in 4 years, but it's quite do-able.
2. What other Ph.D. can you earn without previous familiarity with the field? Ph.D.-level econ is so different from undergraduate econ that it isn't that much of a handicap to start from scratch. You do of course need to know a fair amount of math, but you could have learned that math studying dozens of other fields.
3. What other profession gives you so much freedom to choose your research topics? Many economists now devote their careers to studying topics which an outsider would classify as political science, psychology, or sociology. Some economists even do work that basically amounts to history or philosophy, though they probably need to work on more conventional topics until they get tenure.
4. What other Ph.D. has such a great safety net? A Ph.D. in philosophy (not redundant, though it seems so) who fails to become a professor of philosophy has few good alternatives. An econ Ph.D. who fails to become an econ professor can become an economic consultant and make big money.
5. The pain of the econ Ph.D. is not all that bad. Yes, it hurts at the time. But the key question too many grad students in econ - including me! - lose sight of is: Compared to what? Does it hurt more to be an econ Ph.D. than a Ph.D. in math? History? Classics? Does it hurt compared to a real job? I think not.
Overall, the econ Ph.D. is such a good deal that I would seriously advise people who want to do research in political science, psychology, or even history to just get an econ Ph.D. and become a professor of economics. Even if you have to research topics you don't care about until you get tenure (and you probably won't have to), you could easily earn tenure in econ before you would have defended your dissertation in another field.
Needless to say, it's not really in my interest for my field to be flooded by the best students in political science, psychology, history, and beyond. But - besides giving me the opportunity to do the work I love, with reasonable material comfort, and zero job stress - what's my field ever done for me?