I also first thought it was about water-showers and wasn't going to read the thread because I was like, "Eek, that sounds like an awkward situation!" Then it suddenly occurred to me there were other things called showers...
If you (general) have agreed to let your in-laws throw you a shower, I think you can expect to meet relatives and family friends you've never met before: cousins your SO hasn't seen in years, parents-in-laws' friends who watched your SO grow up, that kind of thing. But, I could imagine getting an ooky feeling from the hostess, if she's saying things like, "We must invite the Smiths, they invited us to their granddaughter's graduation seven years ago. And what was the name of Great-Aunt Betty's son's daughter? She must be about your age now," and the guest list just starts to balloon with people whom the hostess is using for social "payback" or doesn't even know well herself. That would be uncomfortable.
To me, "friends of friends" seem different, and a little weird. Like, Betty and Bob are getting married, and Bob's friend Cindy is invited... and also Cindy's friend Debbie, who would barely know Bob on the street and has never met Betty. If I were Debbie and got the invitation to Betty's bridal shower (meaning I would also be getting one to her wedding), I would be thinking, "That's a little odd. Are they having trouble filling seats at this thing, or what?" I mean, if I don't really socialize with Bob at this point, would that really change after he marries Betty? And even if it does--there will surely be plenty of time after the wedding, when all four of us hang out together, to get to know Betty. I don't need to start the friendship by giving her two nice gifts (shower, then wedding). That's just my take on it, of course.