It's interesting because I think 2nd showers are getting to be more the norm, whether that's "right" or not. And I think part of it is that so many of the "exceptions" are becoming more the norm, too (like, this kid has same mom but dif't dad and his family wants a shower, etc.)
I'd even add one more "exception"--a friend's husband decided to leave her for another woman while she was 3 mos pregnant with son #2. We threw her a shower more for showing our continued support of her and helping with her growing family vs. "welcoming" her to motherhood. (Look, we couldn't have a "let's slash his tires" party.)
I mention this because I've been to numerous second showers and usually they fit into a gray area, although some have just been the "dif't gender" category. And I haven't minded (except for one but that was a dif't story) going to any.
Still, for me, my BFF asked if I wanted a shower or sprinkle for DD#2 and I declined, explaining that you really shouldn't have a 2nd shower and she was a bit surprised, because it's become so normal now that it was just assumed she'd throw another one for me (she threw #1.)
But another friend I know is trying to get pregnant with #3. She's had 2 chemical pregnancies and is struggling. She has 2 sons & wants a daughter. She was in the military and had no shower for the boys. If she actually gets pregnant AND it's a girl, I'm totally going to throw her at least a sprinkle, because I know her journey, and I know she'd love it. That's a good reason to throw her a party, I think :)