Because DD is a girl, she is likely to get pregnant and breastfeed, and I like that one of my children will go through experiences that have been a major event in my life. Obviously she may not have children, but if I had all boys there would definitely be no chance of one of my children getting pregnant.
Another issue is that, depending as well on the gender or cousins and close friends, having a boy and a girl means that they know someone of the same generation very well who is a different gender to themselves, which means they may have less issues about gender (depending on the family of course). I have met people who had no sibling of the opposite sex and they have said they felt nervous/anxious/lacking in confidence about the opposite sex as a consequence.
For me, it also gives me more understanding of what it is like to grow up male and female in a society rather different to when I was growing up, and that gives me a better understanding than I would have otherwise, but also worries me.
I also think it reduces the extent I've got to be the example of female or DH has got to be the example of male because there is another male and female in the house! So it gets away from a notion of males are like my dad and females are like my mum. It gives more examples of relating to people of the opposite sex.
I suppose it is a bit like I'm glad I have a gay sibling. My experience of life and my understanding of gay rights would not be what it is if all my family were straight. Part of that is empathy and intuition though. A more empathetic and intuitive person than me wouldn't need such direct experiences, and maybe you develop more empathy from having less variety in family members? I don't know.
That said, I think people miss out if they're a man who grows up without a brother or a woman who grows up without a sister. So there are pros and cons to any situation.