] One point not been raised yet is that often the so-called "patron" is obliged to promote/recognise the connection even if they have nothing to gain from it, or it might even be an embarrasement to them. To otherwise would be the height of bad manners.

I think it's a little stronger than simply good manners in some contexts. In the Darcy/Wickham situation, it's an unpleasant duty that goes with being head of the family.