The reason for NOT putting registry information in the cards is because a registry is supposed to be a guide. Not a list of demands. People can buy whatever they want with their money. If they don't buy something off the registry that's fine. If they do want to buy off the registry they'll ask.
So no, actually it's not rude. Just RSVP and buy a gift. If there is no registry listed then you can do whatever you want.
Sorry, this is just going to be one of those "agree to disagree" scenarios. If I don't know the MTB, which happens quite often, I want to get MTB something she wants/needs, which is the point of the whole registry. To me, the registry is a guide. And if I don't have it, then it makes my job difficult as a guest to try to figure out what in the heck to get MTB. I find it rude to your guests that you would make them work to find out this information (ex. ask the hostess and/or search stores until I find a registry).
But I will say, that yes, I have bought items not listed on the registry, but that was for close friends. If I'm not close with the MTB, I just want to find something on a registry, buy it, and be done.