The fact it has survived so long - and not been eradicated by evolution - indicates it has helped the human race become stronger.
"I have received e-mails from ex-sufferers saying in retrospect it probably did help them because they changed direction, a new career for example, and as a result they're more content day-to-day than before the depression."
Aristotle believed depression to be of great value because of the insights it could bring. There is also an increased empathy in people who have or have had depression, he says, because they become more attuned to other people's suffering.
a) what evidence is there that (mild) depression decreases sexual fitness?
b) what evidence is there that depression isn't disease relatively unique to the last (say) 50 years?
Depression is referenced in literature dating back before the last 50 years. Goethe's _The Sorrows of Young Werther_ is one example that comes to mind.
The first quoted sentence is fishy. Depression was most likely irrelevant to the survival during the time periods where evolutionary pressures occurred, as it wouldn't have manifested itself. Present day hunter gatherer tribes have no incidence of depression, and the avrage caveman hardly needed "changes in direction" in the modern sense of the term. It couldn't have been selected for directly.
If anything, depression will be selected against if occurred in the early days of humanity. Depression is associated with erectile dysfunction in men, and depressed people are inactive and too risk averse. Not much chance of survival or breeding, I think.
Depression also gets a brief mention in Descartes' Meditations and in Vasari's Lives of the Great Painters, but I am not sure if it is the modern form of depression they are talking about or just melancholy.
I'm not an expert so I'd recommend eg Nesse's article at http://www.huxley.net/rankmood/adapt.htm. My understanding is by analogy to pain. If you were in imminent danger of being damaged, you would expect your pain response to override less urgent concerns. Depression is one's response to damage of a more subtle, social, longer-term nature. For example, not achieving reproductive success, or efforts benefiting competitors rather than kin. If the problem were long-lasting and/or severe enough, you'd want your sense of it to force you to change strategy.
This old idea may be finally finding acceptance. See for example http://www.huxley.net/rankmood/adapt.htm.
If you see depression to be a kind of pain, it's hard to imagine why anyone ever believed the converse: that an ability to feel depression in appropriate circumstances is not an adaptation.
My guess is that this is partly because it requires an understanding that what is good for a gene's survival is not necessarily pleasant for the gene's carrier, and partly because if depression stops being considered a disease, the medical industry stops making money out of it.