No.
First, because for very formal weddings, RSVP cards are bad form. People who routinely engage in formal social correspondence have their own social stationery and prefer to use it, writing out longhand that "Miss Aspasia Phipps/accepts with pleasure the kind invitation of/Miss Jane Lastname...." An RSVP card suggests to such guests that they don't know how to respond properly, so it's offensive.
To people who don't routinely engage in social correspondence, RSVP cards force them to use a form of communication they aren't comfortable with. These are people who routinely use text, voicemail, telephone and email; and in many cases the pleasant walk down to the postal pillar to mail the card is more annoyance than pleasant walk.
Brides swear by RSVP cards because they make record-keeping easier. With only twenty guests you don't need extensive record-keeping, and even those who do can hardly be said to be being gracious by forcing their guests to conform to their rolodex format.
Stationery companies swear by RSVP cards because they get more of your hard-earned money by selling them to you. And remember, most web- and magazine-etiquette mavens are sponsored by such members of the "wedding industry".
But Miss Manners -- the figurative one, as well as the one trademarked by Judith Martin -- will be quite happy for you to leave them out.