Mr Bennet stayed at home without any ill effects to the Assembly, therefore I must presume that it was not absolutely compulsory to attend even when your own wife and daughters did. So why did Darcy go? I can't imagine he would attend an Assembly in Lambton, if they had them there, as (I recall reading this somewhere, but I can't remember where) he would have had to mix with a variety of social classes including those in trade.

I can come up with no convincing reason for him to attend aside from a desire to protect Bingley who has a tendency to fall in love with every pretty girl he meets (but am I allowed to say that since Darcy merely implies it until much later? Or perhaps I only see the implication because I know he says it?)