Wow.. what a bizarre notion! Of all the things that went through my mind when we decided to have children, that was certainly never one of them!
The closest I would say we have ever come to that kind of thinking would be when deciding when to have a second child. It certainly wasn't the dealbreaker or anything, but we thought it would be nice for our child/children to have someone to share the joys and burdens or growing together. My father had an industrial accident about a year ago, and my siblings and I were so grateful to have one another to help with this situation since my mom and he are not together anymore and there was no one else. If I had to do that myself it would have been incredibly difficult. That isn't the only reason I'm grateful for my siblings, of course. I certainly don't think that my kids won't forge friendships outside of family, but there's something comforting about knowing I won't leave my girls "alone in the world." It's not entirely logical, I suppose. But again, that certainly wasn't our primary reason for having more, and we certainly wouldn't have had kids in the first place to take care of us! Ick!