I've declined every invite for a shower for a child 2nd or higher. I give "welcome to mommyhood" gifts once. After that, you're already a Mom and on your own.
I get it, and believe me I'm not just trying to justify it for myself (or maybe I am, ok I am). But what about if you never gave a first gift or if gifts weren't a requirement and the shower was just "come eat some food and hang out"?
Well, then it's not a shower. Again, SHOWERING the honoree with gifts. There is no other point to a shower other than to give gifts. Otherwise, it's just a party. Just invite some people over, and don't make it about the baby if all you want is to see everyone before you have a newborn.
However, like PPs have said, you're trying to find ways to justify it. You feel your circumstance is special and extraordinary, and it just isn't. Bottom line is that a baby shower isn't some rite of passage. It's a two hour get-together with some finger foods, games no one really likes to play, and presents. It's not a life-changing event. You can be disappointed, but at this point, there's nothing to really do about it.