The printers would not be offering these cards if there was not a demand for them and I think they are a very good idea. In our own family, our Golden Wedding celebration, a family week away will see a full house because we told everyone a year in advance. We have just sent the invitations out confirming it and the location.
A month ago two members of the family, planning a wedding, small one for a mature couple, found the only dates available for the venue of their choice was late May this year or next year sometime. They have opted for the earlier date, but know already that one close family member cannot be present because they have a commitment on that day that cannot be altered.