I wouldn't say an R is useless, while they have weaker stats they do well with their abilities to support your team if you have 2 or so SR in there.
At least in the events so far I've never had problems with a team of mostly Rs if the element has advantage. If you have time/resources it's worth leveling all your Rs to 50 just as a fallback, that's what I try to do with all the Kamihime I get