Obviously, you should want to have another... not for your son, but because you want to have another child. Sort of hard to describe wanting though... I always just sort of "knew" that I wanted 2. I was the youngest of 3, the only girl. I felt picked on, etc as a child.. but it didn't make me not want to have 2 kids. I was very close during the summers of pre-teen years, with the younger of my two brothers... we were best friends all summer - played one on one baseball, basketball, tag football, you name it. And board games (3 day long Monopoly anyone?). We aren't all that close NOW. Just practically speaking.... we live 5 hours apart, I'm married with 2 kids, he is recently married with none, so far. But I wouldn't trade having him for anything. No one on this earth, my wonderful husband included, will ever be able to share in the memories of my childhood from a similar perspective! When our parents aren't able to be on their own any longer, I will have someone to consult with about how to best handle things... someone who isn't biased because they are THEIR parents too. I won't feel like it is all up to ME to figure out. In the wee hours of Christmas morning... it wasn't my parents' room I went into to wake someone... it was my brother's room... When I was in college and I was "finding myself" my brother was a phone call away, and he would call me sometimes too, to have a heart to heart about how what I was doing (neglecting my school, debating eloping vs. a "real" wedding) was affecting the rest of the family.
My own kids are almost exactly 3 years apart. Boy and girl. And they are the best of friends. Obviously that is not always the case for siblings. And of course, they sometimes fight and hurt each other's feelings deeply. But they make up and move on, sort of like grown-ups. It's one of those few relationships in the world that you can feel "safe" in... that no matter what you do/say that person will not leave you or hate you forever (generally speaking). I think THIS is the sort of thinking that is behind the quote you mentioned. It really is a blessing.
One of my best friends is an only. She never has had anything negative to say about it. She is very outspoken and sure of herself. TONS of friends. She is married, but childless, however. So her parents have no grandchildren, and never will. Now, that may sound selfish, but at the same time, she and her husband are missing out on an awful lot, too. She fawns over my kids when we see her, but as my kids get older, they are less comfortable because they don't see her that often. She has always been the type to "adopt" her friends kids and spoil them. My own kids call her "Aunt __" and she loves it. But the truth is, she isn't an Aunt. She would be a spectacular one, but for the fact that she has no nieces or nephews.
My kids play together and expand their imaginations together in ways that I (as an adult) cannot relate to. They tell silly jokes that THEY think are hysterical.. and my husband and I can't take it anymore - they were on a knock-knock joke kick just yesterday... for HOURS.... THEY were having fun... I was on my last nerve.
The bottom line is: you just don't know. They may have the best sibling relationship ever, or not, like some of the other moms mentioned. Have another child, because you want one. And teach your older one how to love/share/mentor the younger. Seeing my children together playing and making each other laugh and smile, makes my heart want to explode sometimes. I don't get that same feeling when they are playing with friends on playdates.