I've seen birthdays invitations where parents would put down their children's wishes, much as what's their favorite theme they like, much as Cinderella, Pokemon, Blue Clues, Barney etc. so that give guests ideas of child's favorite cartoons or Disney characters, and make shopping easier. I really like registries, I don't even live with any of them to know what they have or don't have, that's why I rather to have registries to be enclosed in invitations, It does make it stress free for me and its a lot easier knowing where to go. But, if the items are too expensive for me to afford, I would give them a gift card of the store they registry at.
The point of a registry in my opinion is to provide suggestions for those who don't know what to buy, It is a choice and only you can make that choice. I know some people think that a registry is rude, or any suggestion on an invitation is rude, but you can ignored it, and do what you want to do. It's simple as it is.
I know there are plenty of people who are very old-fashioned, still think it's tacky to put any suggestion on invitations. But, I'm not even old-fashion, so it doesn't even bother me at all. I've been told many times that my gifts were the best that's because I've always shop at someone's registry or shop at their wish goals, I make it happen for them.