I just have the one...and probably will for many years to come.
Would she like a buddy? Sure she would. But she doesn't need one. Time, money, and my temporary geographic instability kinda prevent me from getting another dog. I like to think that the extra attention she gets from me can partially make up for lack of a playmate - she doesn't get bored too much. And she doesn't have SA.
I'm not convinced that adding another dog would add much to my life. I get everything I need from the one I have - she's a great companion, and up for anything (and I mean anything!) I want to do from cuddling to long hikes to agility and disc, nosework, swimming, tricks, whatever. I'd want any dog of mine to enjoy the same sort of stuff, and I definitely don't have the time/money to do all of this with another one.
ETA: Looking at the author's reasons in particular...
1) Social interaction. I take Kit to the dog park occasionally. She gets along fine with the other dogs, but mostly she wants to meet all the humans, see how many treats she can beg off people, and chase the ball.
2) Boredom. Kit gets a stuffed treat toy in her crate. She will chew on it for a couple of hours and then go to sleep. She has never been a destroyer of things.
3) Exercise/fitness. If you exercise you dog, this doesn't apply.
4) Sleep cycle. Again, if the dog is exercised when the human gets home from work, it's a non-issue, because the dog is tired by the time bed time rolls around.
5) Health benefits for humans. Study after study shows that pet ownership improves mood, blood pressure, and health in general. But I've certainly never seen any scientific evidence that there's a greater benefit if you own multiple pets.
I'm not trying to poo-poo having multiple dogs; if you want to own multiple dogs and can successfully pull it off in terms of time/money, then great. But I don't think there's something necessarily missing from the life of an only-dog. When I get another (and yes, that's when and not if), I'll do it because I want another, and not because my current dog is bored, fat, anxious, or anti-social.