Immediately after my DD was born, I wanted to give her a sibling. We even tried for almost 6 months, with no luck. Since we have started taking a break, and the new mommy hormones are wearing off, I have been doing a lot of soul-searching about the reason why we were TTC, and when/if we'll start trying again.
I am not sure that wanting your child to have a sibling is enough of a reason to have another baby. I am inclined to think that the motivation has to come from the mother and father themselves, not from a desire to give their other child something.
I have a sister, and I have a great relationship with her. She is currently trying to conceive, and we are hoping it happens soon so that Katie will have a cousin near in age to her. So my feelings aren't based on the lack of a good relationship with a sibling.
However, there are a lot of things to think about, beyond just providing a sibling, and I don't think that that one thing should guide a decision that important. For instance, my DH is a teacher, and we can barely afford our current family lifestyle, let alone the addition of another child. We want to be able to send our DD to college, take family vacations, etc. someday, and I can't imagine how we could afford it with two. Also, my DD is the light of my life and the center of our world. I hate to think about the possibility of missing out on a single thing in her life, but when you have two children, it is inevitable. If both children have events on the same night, you can't be in two places at once. Sure, you can take turns or something like that, but I want to be there for everything, which just isn't possible with two children.
On the other hand, I love the baby stage, and as DD is growing out of it all to quickly, I start thinking about how nice it would be to have another baby around. Also, there are some things I missed out on with DD, and I'd like a chance to do those things differently with another baby.
So, I'm still up in the air about it, but I do think that there is a lot more to consider than just wanting a sibling for your first child.