In the NYTimes:

This radical idea — the scientists were suggesting that depressive disorder came with a net mental benefit — has a long intellectual history…

Andrews and Thomson see depression as a way of bolstering our feeble analytical skills, making it easier to pay continuous attention to a difficult dilemma. The downcast mood and activation of the VLPFC are part of a “coordinated system” that, Andrews and Thomson say, exists “for the specific purpose of effectively analyzing the complex life problem that triggered the depression.”

And:

“The high relapse rate suggests that the drugs aren’t really solving anything,” Thomson says. “In fact, they seem to be interfering with the solution, so that patients are discouraged from dealing with their problems.["]

One solution:

“What you’re trying to do is speed along the rumination process,” Thomson says. “Once you show people the dilemma they need to solve, they almost always start feeling better.” He cites as evidence a recent study that found “expressive writing” — asking depressed subjects to write essays about their feelings — led to significantly shorter depressive episodes.

Here’s the original paper referred to in the article.
The NYTimes article is also posted at the author’s blog, The Frontal Cortex.