Sabina was my second four-star and first four-star healer. She was a long-time staple of my lineup and still gets plenty of use at +18! Yes, Rigard is better, but if you are like me, it could take a long time to pull Rigard. Plus, Sabina is (I think) the only healer available for sorcerer class trials, which can make a big difference. I wouldn’t hesitate on her at all.
All that said, I assume someone has shared with you the value of leveling two rainbow teams of three-stars before you get too carried away with four-stars. If not, see my standard speech on the subject here (mostly the first couple of paragraphs). If you’re already running into ascension material issues, you should well be able to appreciate the value of a deep three-star bench, given where you are in the game.