I would say the main reason for waiting is medical. The rest comes down to preference. I read this article which lists most pros and cons including medical for different spacings. I found it very interesting:
http://www.thealphaparent.com/2012/07/what-no-one-tells-you-about-child.html?m=1
I had all my 3 kids back to back n now pregnant with my 4th child.it's hard at times but the end of the day it's all worth it!!!
This will be my first, but I was thinking the same thing about having another right after I give birth...possibly waiting 6 months to start trying again. I always hear mixed reviews bc there are definitely pros and cons to having them back to back. I know they say waiting is much easier and healthier for our bodies, but my dr did tell me if I am determined to have them close make sure to wait at least 6 months and continue taking your prenatals. Good luck!