I think that many researchers do look at how partial expression of traits could have some benefit to the organism, the issue is that redefining a term like 'depression' has little merit to it.
Arousal is a good example, we can't spend all day in a state of arousal, if we are then we become 'stressed' rather quickly. So it would appear that there are mechanism, mostly conditioned , that create states of arousal in humans. Obviously the arousal state labelled 'fear' is going to have certain states where it occurs and the body organism is going to enter high levels of arousal. This can be very beneficial if you have to fight or escape.
However in some people this level of arousal occurs in odd situations that seem to have little benefit to the individual. Phobia with panic attacks being a good example.
Now there may be reasons why some people have strange things have to their circadian rhythms on a seasonal basis. If one lives in a higher latitude there could be benefit to states of sleepiness during winter and less sleep during the summer. But it would be really hard to parse that out from the behavioral components, but it could have an actual mechanism related to the length of day.
But just disturbed sleep cycles are the greatest reason people seek treatment, insomnia has many variations and forms, all just related to the disruption of the normal sleep wake cycle, not even getting into the delayed sleep phase people.
So if there are complex mechanisms in place that regulate levels of arousal, attention, sleep/wake cycles there are a myriad of possibilities for partial aberration that have either some minor benefit potentially or some minor detriment, say the large number of people in families with bipolar disorder who have hypomania. They are productive, they work long hours, they get by on six hours of sleep, now they may have some detriment in low impulse control and addictions.
However these traits as partial expressions are one thing, the same traits in full expression that produce florid mania, psychosis and impulsive acts of a detrimental nature, often with little regard for danger or conversely severe vegetative depression or psychotic depression, these are not beneficial to the individual.
It is even harder to look at possible links to reproductive success, outside of just individual benefits and detriments.
The point being, yes partial traits associated with symptoms of depression could have potential benefits or detriment of a mild nature. However full blown depression is dysfunctional by definition.