I think the fundamental problem is, that those who think subsequent showers are rude, do so because they feel that there is an underlying expectation that they have to bring a gift. And that by the very fact that they were invited to the party is a summons to bring a present. It's all about the gift...even though, if that were the case, they are receiving hospitality in exchange for that gift.
I am one that thinks of bringing gifts to showers. But, I also think of bringing gifts to birthday parties, christenings, graduation parties, housewarming parties, etc. I often think of bringing hostess gifts to dinner parties, or weekend getaways that is being hosted by someone else. I don't bring gifts to these events, though, because I feel obligated to. I don't assume that the reason that hospitality is offered is because the host/hostess wants something in exchange. That's certainly not my goal when I host parties. I bring gifts because I want to. I don't feel etiquettely required to. I don't feel judged if I don't (and if I am, I don't see it as my problem). It's a choice I make. I have some amount of love/affection for those I give gifts to, so that's what I do. If I'm invited to a party and don't feel close enough to the person to bring a gift, or don't feel that a gift is warranted, then I don't bring a gift and/or don't attend the party. It's not some huge moral dilemma everytime I receive an invitation to a party. It's a simple "this one, yes. That one, no." and I'm done.
I gift every single baby that is born into my family and to close friends. Every one. It doesn't matter to me how many kids have been previously born into that family, what the income level is, or whether or not I'm first invited to a party. And by the baby's nature, most gifts really are something that parents would use to help take care of the baby. It's not that I'm supplementing them by giving them things they should get themselves...it's a gift. Why would I get something that isn't useful or needed? That goes for every gift. And I feel no more obligated to gift the second and third baby as I did the first. There's no sense of obligation in it anywhere.
As for being invited to a shower...if I liked the traditional shower format (which I don't actually enjoy), I think that being offered some cake and social stimulation is a good thing. The gift is already a given. So the party is just a bonus.