Here's the one reason you missed: Customers expect you to be there. When they have a problem, a question or perhaps even want the praise the brand, some consumers will use Facebook as their channel of choice. If you're not there, you will have disappointed or frustrated that customers and possibly weakened the relationship.
This article--like so many about social media--is written from the perspective of a marketer. It's all about ME--what do I and my brand want? What is the value to ME? How much am I selling on Facebook? What's the value of a fan to ME?
Focusing on social media from this perspective is like saying, "Should you close your customer call center? I mean really, how much value are you getting from it? What's the value to your company of each call? Aren't there other ways to contact the company?" Of course, it makes no sense to close a customer call center because the damage will be greater than the cost savings, and the same is true of Facebook, as well.
Yes, EdgeRank makes it damn near impossible for brands to break through the noise and get inclued on consumers' News Feeds, but isn't that really what Facebook users want? No one signs on to Facebook see what brands are doing, so it is and SHOULD BE difficult for brands to get the same sort of attention in news feeds as one's family and friends.
If a brand is smart, it will view Faebook less as a push channel and more of a pull channel, and will not overlook its value in customer service rather than only frame it as a marketing one.