Well clearly it depends on the relationship between the siblings but assuming that there is a good relationship between them, or at least a passable one, then I'd always think having a sibling makes life an awful lot more easy in many respects. I'd also say that in my experience the number of people who have good relationships with their siblings far outweighs those who had bad relationships.
In terms of parents, yes, it certainly does make a difference (certainly in my experience). On a practical basis there is someone to share the all the practical burdens, be it simply making phone calls or visits regularly, making sure that someone is with them at Christmas or birthdays or helping organise things for them if needed.
Secondly, there is someone who knows exactly how you feel and what you're going through when times get tough with your parents, as they invariably do as they get older, be it illness or death. Even if you have the best, most wonderful partner in the world, no-one will worry about your parents as much as your sibling (or listen to your worries about them). My dad died about four months ago and it was amazing having my brother around. Not because he did everything (he didn't), not because he was calm and composed (he wasn't) but because both the practical burdens (e.g. wills, death certificates, funeral arrangements) were shared and, most importantly, the emotional burden of coping and supporting our mum was shared.
Finally, no-one has shared memories like your sibling. Of Christmases, of holidays, of childhood toys, of your parents when they were little, of favourite hiding places, of the funny noise in the attic. No-one, not your partner, your best friend, no-one. You have a unique and shared experience that will live as long as both of you and if, and when, your parents die, that will continue.
Yes, you argue, yes, you fall out and no, you don't see each other as often as you should but that doesn't make your sibling any less important in your life.