good, i say go for it. sometimes you just have to say sod off to etiquette. a lot of it is so old fashioned. i've been picking and choosing what rules i want to stick with, as far as the wedding stuff anyways, and for baby showers i feel that you should celebrate each baby!
for one, it's not the poor 2nd/3rd etc baby's fault that it wasnt first. second, as life changes and friends/jobs come and go, the mix of people invited might be comepletely different! so like you, a bunch of guests might not have been there first time around, but would love to celebrate the new baby with a full blown shower. i see absolutely nothing wrong with that.