I've been on the receiving side of this on a couple of occasions. The most recent, one of my dh's nieces was getting married. My dh isn't close to this niece or his brother and SIL, and we actually hadn't seen any of them for years. So when we heard she was getting married, we certainly didn't expect a wedding invitation. We did get a facebook communication from SIL saying that they won't be able to invite us to the wedding, but they would really like us to "share in the family's joy" by attending one of the many showers. She listed the dates/times/themes of four different showers and asked us which ones we would like to receive an invitation to.
I emailed SIL back, congratulating her on her daughter's marriage and wishing them the best, while also advising her we will not be able to attend any of her showers. The bride herself emailed me back saying "okay, but here's my registry website".
SIL then called me and told me how rude I was for not attending one of her showers. I told SIL that etiquette states that you never invite someone to a shower unless they are invited to the actual wedding ** I know work showers are the exception, although I don't agree with that personally, but I don't' let those get to me**. SIL responded that was old fashioned etiquette and that "modern etiquette" says that it is fine to invite people to showers rather than the wedding. "It helps hold costs down while allowing more people to participate", and "people will get hurt if they don't at least get invited to a shower" So you see, some people out there in the world really think this is OK.