Hi iFreilicht, thanks for the warm welcome and the suggestion about seeking verified affiliation, I've submitted a request via the contact form.
As to your excellent question, those vents are in fact intended for passive cooling (which is our particular specialty at Logic Supply). To find out more about why they're spaced in such a way, I went directly to the source, our Engineer Hans who designed and developed the MC600, here's his response.
We've had experience in the past where grouping the ventilation holes has caused some warping of the sheet metal during laser production of the case, so that space between the vents is essentially a 'pause' between areas of ventilation holes. It also matches the input size to the output size to help keep airflow through the case at a reasonable velocity. If the whole side was holes the air would have a tendency to just kind of meander across the mobo space, which is the area our typical customers need cooled most. It also matches the other side which is nice for symmetry depending on your aesthetic opinion.
I hope that gives you the answer you were looking for. While we're primarily an industrial computer (IPC) company, and the MC600 is largely intended to be used in-house to create custom systems for our clients, we greatly value the input of individuals creating and researching small form factor systems of any kind. It's fantastic to see a thriving forum like this devoted to SFF systems.
Thanks again for your question!
~Darek