Luciana Nery said:
That's rare indeed, I believe that most only children would consider having only one child themselves.
There's only one instance I can think of in which it's highly undesirable to have an only child: parents who move frequently and lead gipsy lives. Because we don't have siblings, at least we have the comfort of having friends at school and living in an known environment. When frequent moves are necessary, siblings provide a lot of support to each other.
About being spoiled... I probably am, but then I wouldn't even notice it.
But above all things, I'm a very undemanding and easy-going person. It seems to me that people actually enjoy spoiling me.
And there's nothing I can do about! 
aerosolben: many children with siblings are antisocial too. And while it's true that only children have more difficulty in dealing with other children... at least they have the advantage of knowing how to deal with adults better. When they become adults themselves... won't that be an advantage?
I've seen quite a few researches indicating that only children have better salaries than people with siblings - I can try to look it up if you want to - which is easily explained by the superior education. But maybe their social skills in an adult environment plays a role too? I'm not sure if this has ever been tested.
I was an introvert as a child, but quit it in my teenage years. Not worth it, I thought. People can be fun sometimes.
All of my social skills were developed there, and nowadays I can pretty much adapt myself to any social situation with absolutely no problem. So yes, it can be developed, and it will depend on the person.
tygirwulf: I too have to "decompress" after being around many people.
Loneliness... if anything, I feel powerful when I realize that I don't need someone's bad company just to placate my loneliness. I can do well alone. I see people basing entirely relationships solely on the grounds of having someone nearby. That's so sad.
Funny, I am the eldest of four, and would never consider having an "only child". I realise, however, that there is a strong tendency in humans to defend whatever it was "they went through", because it is an innate part of who they are and thus to admit it is not `the ideal' somehow reflects on them!
That said, your comment about gypsy lives is appropriate for me - my family was extremely nomadic.
Re Education: I disagree - my education was largely due to my parents (both are teachers) and although we had limited resources (I remember my two sisters missing out on a field trip because we couldnt afford AUD$2.50 each for them - it still upsets me remembering) I simply cannot imagine having had a better education. We have all done well (one physicist, one mathematician, one doctor [Ugh] and one engineer). So could I have done better if there was only me? I think we have done well not because we were educated at fancy schools and with unlimited resources, but because we were taught a deep love of learning. My parents could have taught that to another 10 kids if theyd had them..
Re Socially: My siblings are some of my best friends - partly because they were some of the constants in my life. We regularly travel to the otherside of the world just to visit each other. Part of this was due to my mother, who was a lonely only child, in some ways "forcing" this on us when we were very young. I appreciate having so many people in my life that will be with me whatever comes...
Re Aging Parents: We talk about this regularly, although my parents are only 57. My one sister and I bought a house on the beach a few years ago for my parents to retire to (as immigrants they will have no pension, and we all understand that we will be supporting them once they do retire). [You've never seen a more confused real estate agent - he couldnt work out if he was selling to my sister and I or to my parents!] This is definitely one area having siblings will make things much easier.
Finally - siblings are people who have experienced a huge percentage of the life you have. More so than even a best friend you had since you were age 2. There is something nice about people approximately your age who have such a large overlap with your experiences being around - there's so much you dont have to explain. They are people I love as much as my parents - and that kind of love is never diluted because there are more than one or two of them. More people to love in your life just cannot be bad....