Thanks, IB, for posting these here. The consensus of these articles is that the oil gland is important for good supple feather quality -- apart from other benefits. A search of precis for related articles on http://www.pubmedcentral.nih.gov shows the same emphasis. I could not locate any articles in English, Spanish or Portuguese (Brasil) supporting the idea of the oil gland not being related to feather quality -- which does not mean they are not published, only that I could not locate them.
I tend to think the oil gland should be examined when feather quality disappears from a pigeon -- but poor nutrition and/or a drop in general health can affect production in the oil gland, i.e., is it a chicken or the egg situation? For what it is worth.