we just really want the surprise. i've just always wanted that moment where doc holds up baby and says "it's a..." it just sounds fun to me. and i just really don't plan on going overly gender-y on decorating either. i've never been a pink is for girls, blue is for boys kinda person.
someone also brought up a really good benefit that i hadn't thought of initially. when people know the gender they buy clothes. piles and piles of clothes. so many that the baby grows out of many of them before they're even worn. when people don't know the gender, they buy off your registry and you get things you actually need. i'm a very minimalist and practical person, so that's just right up my alley.
but it's mostly for the surprise of it all.