A lot of accounts about why aliens are interested in humans is because they cannot reproduce. This is based on accounts of persons who were abducted. You will also note that many aliens are shown basically as without human reproductive organs or asexual.
For me, I try to understand things in a micro not macro sense. Based on microbiology… ANY intelligent being with our technology can create life in a dish hence them trying to use us to understand how to reproduce does not make sense or I do not buy that scenario.
It is like the Matrix - a great movie with highly aware ideas and philosophy and yet the root reason they gave for keeping humans did not make sense.
For me when something is true all things have to add up unless of course the accounts we get is not true.
Not to discredit these "Credible eyewitnesses" but I believe a certain distinction can be made. That these abductees had sexual acts performed on them would be considered facts or observations. However, I would argue that the assumptions that the aliens are unable to reproduce would be merely that, assumptions.
Taking the account of these 'eyewitnesses' at face value, I think there are some reasons as to why they (The aliens) might be interested with our reproductive system.
Maybe despite being able to create new lifeform in a dish, by cloning, lab-grown vat, etc, there might some inherent flaws/defect in final result. They see us, humans, can reproduce with relatively defect-free offspring. Thus, they are interested in learning more about our reproductive system.
Another reason might be the time and/or resources required in reproducing asexually (By their standard). Perhaps 9 months and two people doing it is a much more effective method of reproduction than what they currently have. Combined with the first possibility, it's easy to see why they might be interested in human reproduction.