I think it's more the 'culture' of the family that determines sib closeness. I love watching my teen and yougest together (10 yrs difference). They melt my heart. The don't have anything 'in common' right now, but it doesn't mean they are not loving and appreciative of each other. My boys are 5 yrs apart and I love that as well. they share a room and a love of music. I love hearing them chat at night. My youngst ds enjoys falling asleep as his brother reads or does homeowrk. he says it's very cmforting. My girls are 6 yrs apart, but they play together every day and also share a room. Right now they are snuggled together listening to a book on tape, the younger watching the older knit.
My two middle chidlren, the closely sapced ones are like twins. It's only now they are doing things without the other. While it's true they are very close and depend on each other very much, all of my children have special relationships, in various ways, with each individual sibs. I think the age difference will matter less as theyget older- i mean what's the big diff between 40 and 50, say? Or 60 and 70?

Anyway- we do not stress competition among sibs, we try to respect individual differences- fi, I've never said 'why can't you be like...." or 'Well, your sister never..." or whatever other awful things some parents say to kids. That's sets up sib relationships for horror, envy and competition. I also made a mental shift when they were tiny to think of my close-in-age babes as twins. I didn't expect the 16 mo old to need me less, or not be a baby because I had a newborn. But that was hard, and I don't recommend it. But I also wouldn't say it's the worst thing a family could do, either. I know a lot of women in their 40's having babies and it's not like they actually have time to space kids 5 yrs apart. If once can and wants to, it's lovely.
I guess I'm one of those who can look back on those tough early times and say it's now wonderful. (Although it wasn't horrible back then). Still, even they were spaced further apart, I think they'd be close anyway, since all my kids get along. the age spacing currently makes it easier for activities, and given they have similar interests. (Lucky me). They share the same 'generation', which helps. They can be in some of the same activites, which meet at the same time and at the same place, be in the same UU sunday school class, go to camp the same week etc, which comforts them and helps with some (not all) logistics.
I think it's personal. I think their are bennies either way. Just as I think there are *tremendous* bennies to having an only. No spacing is insurmountable in this day and age. I think enjoying each babe as they come is very important. I also think it's very important to repsect each child for who they are. But we all here know that already. Looking at my kids now, I wouldn't actually change their past, or our odd spacings. They are wonderful to watch together and I feel blessed.