Is amiability a virtue? I think Yes. Not only is it a virtue, it may be the most important virtue of the 21st century.
I figure the canonical text for thinking about amiability has to be Pride and Prejudice, and what it teaches us is instructive -- indeed parts of the book read like a philosophical treatise. Most obviously, we have The Moral of the Story -- or one of them anyway. You remember Elizabeth Bennet and Fitzwilliam Darcy, in an initially fraught relationship, teach one another a couple of lessons on their way to romantic happiness. And you remember that throughout, Darcy, who is cold and proud, is contrasted with his friend Mr. Bingley, who is the picture of amiability. The simple version might be: Darcy starts off being less amiable, he becomes more so, and this is good.
I think this is right, but it's a little complicated. Because Darcy's qualities are complex. His early qualities present a combination of things. He doesn't ask after people; he doesn't dance at balls; he has total confidence in his own judgment and never seeks advice; and he has a keen sense of his own superiority.
When Darcy changes, he doesn't change all of these qualities. Far from it. He learns to be gracious to people. He asks after Elizabeth's family; talks to her friends -- indeed, makes her family and friends feel valued, welcome and respected.
But he is still a man who knows his own judgment and never seeks advice -- and we think he probably has a sense of superiority. Even after the transformation, Elizabeth's father says something about how even if he had his doubts about the impending marriage (which he doesn't, really), Mr. Darcy is a man to whom one can refuse nothing.
So in this context, amiability need not mean being flexible or accommodating in any deep way. This view is supported by the amusing and philosophical exchange between Darcy, Bingly, and Elizabeth about whether it is better to change one's mind solely on the urging of a friend -- a friend who offers no reasons or arguments one can assess for one's self.
Darcy of course suggests not. To which Elizabeth says, "To yield readily -- easily -- to the persuasion of a friend is no merit with you." And Darcy replies, "To yield without conviction is no compliment to the understanding of either." Elizabeth protests: oughtn't friendship itself go for something?
It might be thought that in Darcy's transformation, this is one of the things that he changes: that he allows the persuasion of a friend alone to have merit with him. But I don't think so. This is, instead, one of Elizabeth's changes. She trusts Wyckam, but when she yields to his persuasion of what to believe, she makes a fundamental error, and comes to see it as such.
So I think the book presents the view that amiability is good, but amiability does not mean listening to your friends and taking their views into consideration without reasons.
But neither is amiability merely a matter of nice manners. It goes beyond nice manners, because in addition to being pleasant, amiability is a way of respecting people. This, it seems to me is the sentiment expressed here. Darcy's problem is partly that he doesn't respect people who aren't in his high class of society. But his problem is also that when he does respect them, he fails to communicate this to them. What he must learn is how to do this, and one does it by asking after people, dancing with them at balls, making people feel welcome.
The reason this is a twenty-first century virtue is that we're sorely in need of communicating respect for others through kindness, attention, and interest. "Respect" for people has somehow come to be associated with leaving them alone, but everyone knows how it feels to be left alone -- it feels awful. And I'd venture to say that the worst days of most people's lives come from being treated without amiability. Someone is just dismissive, or ignores you, or doesn't smile back, or acts like they're in charge or better than you. It's a horrible feeling.
Being amiable isn't always easy, and sometimes it means being kind and attentive when you're in a bad mood. But this just goes to show: it's not a trivial thing, it's an actual virtue. Try it out for yourself: at the mall, at the post office, at work, even -- gasp! -- on the internet. You'll make the world a better place.