I know that etiquette would consider it rude to include reply cards.
I know that etiquette barely cares about this issue, not only because most people do it, but also because most people do it and guests usually actually like it.
And I know that this deviation was only partially driven by stationers and paperies pushing more paper, with it's main motivation being frantic brides, clueless guests, and modern times...in which people don't seem to have everyday stationery or ability to mail things that aren't pre-stamped.
So now it's normal to include reply cards and very few people would think it's rude.
Do you think people are so used to having them, that they wouldn't be able to figure out how to reply without one? I don't mean to suggest that people don't know how to reply (which is what reply cards do), but I know that people get a little edgy about everything wedding, and might go into a panic over how to respond.
If you received an invitation, that was clearly addressed to the invitees (about as clear as any other), with a return address, that even had rsvp on it...would you have any reservations with mailing a response on your own?