Hello! At least this time, I'm around for it. Though by now I honestly don't think anything I say will matter, because most people are dead set on seeing Rosa as worthless. I at least feel better actually showing that while most people don't care, at least I do. Bear in mind, I'm a Rosa fan, and defending her as a character is at the heart of defending her in terms of her relationship to Cecil, ESPECIALLY after the way you framed her (which is, really, how everyone frames her these days).
I'll link you to this first.
Translation of Final Fantasy IV Documents on Settings Book/Settei Shiryou Shuu/Compendium/What Have You
Firstly, Rosa and Cecil have known each other since they were children. Their ties have built up over all those years. This is the most obvious reason. And as you can see in Rosa's profile, she's highly sought, yet her love has always been for Cecil.
I now have to go into the events of the game because you don't get to see much of what happened before.
The opening, with Rosa coming to his room, the things she says, implies that she's been one of the influences to keep him from fully succumbing to his dark side. She's obviously not the WHOLE reason, that would be absurd to say, but it's one of the reasons. She's a reminder of who he was in a way Kain cannot be. Kain is another hardened warrior; Rosa is a nurturing healer.
My next one, which people LOVE to disregard: the gaps. The most important gap of all is the time between Cecil leaving Baron and Rosa showing up in Kaipo. Notice she arrives alone. Notice she would've had to travel across land, mountain and desert to reach Kaipo, and out of all of that, the worst she has is Desert Fever, an illness anyone could have contracted through prolonged exposure to the desert heat. She does this because she loves him so deeply that she wants to make sure he's okay, and she wants to be by his side to ensure it. She isn't one of those wussy types that hides back home worrying herself to death when she knows she could actually be on the front lines beside him, despite the risk. That's how much she loves him, willing to risk her life to make sure he's fine. Though yes, I know most people would prefer she didn't exist at all, her contracting the fever isn't as bad as some people suggest.
The second thing I have to mention is her abduction in Fabul. Sure, she got abducted. And guess what? It happened right in front of Cecil. How is a white mage supposed to fight back when a martial artist and a dark knight can't? This is a bit skewed because the new version of the scene just has Golbez abduct her without her catching on to it happening until it's too late, but the point still stands: there's nothing she could've done. More importantly, if she HAD been able to and HAD done something reckless, that could've easily led to Kain or Golbez simply killing them all off. Which leads me to... how she stops Kain from killing Cecil in the first place. She was on duty healing soldiers. It's apparent that in the gaps somewhere, she learned that the monsters had reached the crystal chamber, and she rushed back there to try and help him. Again, the depths of her love on display. When she's needed, she'll be there with her healing magic if it's at all in her power.
Third, and people love to overlook this: Mt. Hobs. Sure, they remember Rydia got over her fear of fire and melted the ice. But they forget that it was primarily Rosa, with a few lines from Edward, that Rydia got up the strength to break past that fear and open the way. You can't do it without Rosa, she's the key. This alludes to a gentle, kind, nurturing spirit that you don't get to see much over the course of the game, sadly. Yet, it's there, if you bother to look.
Now, I've talked about in-game. Here's outside the game.
Outside the game, in the real world, Rosa was the start of what we have now come to see as the "white mage stereotype." The kind, loving, soothing, nurturing woman. She's also the lead female, and like other lead females, she needs to be helped. There are two things here I need to address.
The first, the "white mage stereotype." I don't know which game you started with, and I'm speaking on general terms. Most people started with FF7. This was their introduction to white mages, through Aeris. They then played 8, 9, and 10, and each one had a lead female/white mage type. So here's the problem. After playing these games, the player then goes BACK to playing FF4, the game where it all started... back when all the details weren't fleshed out to the point where you could accept those traits as gimmes and go on to the other traits that the character has. So how does Rosa look in comparison? "Generic." There isn't much emphasis on her noble background, because there's greater emphasis on her loving white mage traits, ones people know to a T today. People are essentially giving Rosa bad marks for having the nerve to be the first one, but not the popular one.
The second, the "damsel-in-distress." Have you bothered to think her through objectively, next to the other leading ladies you have to help? Rosa only needs your help twice throughout the entire game. Aeris needs your help at LEAST twice, if not more time. Garnet has at least three problems (plant monsters, something I can't remember where she ended up having her eidolons taken, and the loss of her voice). Yuna needs help at least three times, thanks to abductions. And, of course, Rinoa. She's not any bit worse than any of the others, it's just that the game isn't as expansive in its script, so Rosa doesn't get a chance to show off her other traits in as outspoken a manner as the other girls.
This is practically a book now, so you probably won't read it, but it's everything I can think of to say right now. There's a lot more to Rosa than you and most anyone else gives her credit for, you just have to look. Will you? Probably not, nobody ever does. I still feel better defending her than letting it run rampant.