Well, my kids are so close together (1 year 3 mo difference) for a few reasons.
#1 It took us about 5 years to conceive my first (3 years of no luck, 1 miscarriage), which was much longer than we expected. There were times when we thought it wasn't going to happen, but in the end, Clomid shots seemed to do the trick. Because of the trouble we had, we decided not to wait around for our second (I was pushing 31, my wife 33 and really didn't want to wait around anymore). We ended up pregnant on our first try without outside assistance. Go figure.
#2 I come from a family of 4 and my parents meticulously planned us all 4 years apart. Honestly, I don't think it was that great. My parents said to me a few years ago that it's hard because you get out of the mindset of being in 'baby mode' just to be forced back into it, which they found difficult. It's a lot of stress doing two young babies at the same time, but since they go through milestones together, you're never having to switch gears in such crazy ways.
#3 The last problem was, I was never able to be that close to my brothers or sister mostly due to age differences. They'd hit milestones and I just couldn't relate or I was too young to fit in (or too old). The minute one of us would hit middle school, the other hit high school, hit high school, the other graduate, etc. I still get along with my siblings, but honestly, I talk to them once every 6 mo or so now, if that. I just have very little in common. On the flip side, my wife is about a year apart from her brother and they're really quite close. They never fought the same way I did with my siblings (we really fought a ton) and they have a lot in common because they did so much together (both in the same high school band, took similar class trips, shared group of friends). Personally, I'd rather my kids be a little closer relationship wise, especially since it'll just be the two of them. When my parents die, I may never talk to my siblings again, or at least, very sparingly. My wife's brother comes over a few times a year just for vacations and I don't see that stopping really.