Thinking about this again after a conversation with Professor Maciej Henneberg from the University of Adelaide. At present, people only stay in space for relatively short periods of time. (The longest occupation was a USSR cosmonaut who spent over a year on Mir in 1994-1995). There are many health issues related to living in space; one of the major risks is prolonged exposure to cosmic rays and other high energy nasties.
So in terms of assessing the long term effects of this exposure on human tissues, we're lacking serious longitudinal data.
But wait! Perhaps we're not .... you see where I'm going with this. Studying human biomolecules from liquid/solid waste ejected during crewed missions could supply us with that data, without having to risk the health of actual people! The very fact that DNA and other complex molecules would have been degraded by high energy particles is not an impediment to scientific research: it is in fact the very information that we seek.
Of course there are many other factors to consider ... location and retrieval, dating the length of exposure, the source mission, the likelihood of survival in LEO, etc etc. We know that Mir was surrounded by a halo of frozen urine particles, and returned aerogel surfaces have preserved yellowed traces of impacts with it ... so perhaps this is the best way to obtain the samples.
Someone will be persuaded of the value of this argument eventually .....