I think that so many people believe all the stereotyping that surrounds only children:
they will inevitably be spoilt, won't share, be selfish, be oddballs who can't socialise, throw hissy fits when all the purple Quality Street are taken etc...
having siblings doesn't give you the moolpoly on being a well-balanced paragon of virtue. Nor does it mean that that you won't be spoilt/nasty/rude/weird/selfish *delete as applicable (or even all of those things). Some of the oddest and most obnoxious people I know have siblings - and some, plenty of them!
only children are lonely children...
not necessarily true - people just imagine they are. In fact, many children who have siblings are lonely because they get bored to death of only having their sibligs to play with. As someone here has already said, one of their reasons for having another child was to "create a ready made playmate" for their existing child. They can get bored to death of one another and sick of the sight of one another too - it's also hard for the younger one as the older one grows up and doesn't want little brother/sister in tow anymore. It's hard for the younger one to take the rejection and they don't understand it when they've been welded together for x number of years.
They carry all the weight of parental expectation on their shoulders...
That totally depends on the parents and can be just as true of children with siblings. If you're the sort of parent who is inclined to boast and brag to friends and family about your kids' achievements and live your life vicariously through them that's going to happen to your kids anyway - no matter how many of them you've got. If like me you're laid back and not that way inclined at all, you'll be like it with only one child, or 10 of them. Luckily I don't judge success on how many piano gradings my son has or whether he obtains 20 A* GCSEs or makes it to university.
They will always have someone when they grow up...
There are countless brothers and sisters around who literally can't stand the sight of one another and would quite happily never wish to clap eyes on them again given the choice. When children grow into adults they make their own way in life and make their own friends and relationships. Who is to say that my only son won't end up in a relationship with someone who has a huge family or that he won't decide to have 8 kids of his own and create his own huge family? He wouldn't be alone them would he? No one knows what the future holds and you can't make any assumptions about it.
They will be able to look after us together when we're old...
In these days of ageing populations you're never more than 30 feet away from someone who moans endlessly about how they look after their ageing mother/father tirelessly with absolutely no help whatsoever from their miserable, uncaring, selfish bastard of a brother/sister or brothers/sisters.
And finally, I'd happily swap my selfish useless cow of a sister for bags of money tomorrow if it could be arranged. I'm sure there are many out there who would happily do the same too